Screening and diagnostic testing for HIV infection in adults

INTRODUCTION — Approximately 13 percent of persons with human immunodeficiency virus (HIV) in the United States remain unaware of their HIV status, leading to significant morbidity and risk of transmission to others [1]. Improved access to HIV testing and new testing algorithms that enhance the detection of individuals with newly acquired infection can lead to increased access to antiretroviral therapy (ART), which can decrease the number of individuals who present with advanced immunocompromise and reduce transmission to others. Patients without other significant comorbidities who are treated before significant immunosuppression can expect a life expectancy approaching that of the general population.

This topic will address screening and diagnostic testing for HIV in adults in clinical care settings. Detailed information about screening for HIV in the blood supply, the diagnosis of HIV in children, and the diagnosis of HIV in patients presenting with symptoms of acute infection are found elsewhere. (See "Blood donor screening: Laboratory testing", section on 'Infectious disease screening and surveillance' and "Diagnostic testing for HIV infection in infants and children younger than 18 months" and "Acute and early HIV infection: Clinical manifestations and diagnosis".)

INDICATIONS FOR TESTING — HIV testing should be performed to diagnose HIV in patients with clinical signs and symptoms of acute or chronic infection as well as those with a possible exposure to HIV. Screening for persons who are asymptomatic should also be incorporated into routine care for all adolescents and adults. (See 'Routine screening' below.)

Signs and symptoms of HIV — All patients with signs and symptoms of acute or chronic HIV infection should be tested. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Clinical features' and "The natural history and clinical features of HIV infection in adults and adolescents".)

For patients presenting with clinical manifestations of acute HIV infection (table 1), testing for HIV ribonucleic acid (RNA) should be added to standard screening tests. A more detailed discussion of how to diagnose acute HIV is presented separately. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Diagnosis'.)

Possible exposure to HIV — Patients who present after a high-risk known or possible exposure to HIV (sexual or percutaneous) should be tested for HIV and assessed for post-exposure prophylaxis. Such patients should then have follow-up testing over the course of four to six months (depending upon the type of test that is used). Topic reviews that discuss the approach to HIV testing after a possible occupational or nonoccupational exposure are found elsewhere. (See "Management of nonoccupational exposures to HIV and hepatitis B and C in adults" and "Management of health care personnel exposed to HIV" and "Management of nonoccupational exposures to HIV and hepatitis B and C in adults", section on 'Follow-up HIV testing (regardless of HIV PEP)'.)

HIV testing is also indicated for the following groups who may have had an exposure to HIV:

●Individuals who seek screening for sexually transmitted infections [2]. This provides an opportunity for risk-reduction education as well as diagnosis of HIV. (See "Screening for sexually transmitted infections".)

●Persons who are considering antiretroviral medications for pre-exposure prophylaxis (PrEP) as an HIV prevention strategy. For such patients, HIV testing should be performed prior to starting PrEP and every two to three months thereafter. PrEP is discussed in a separate topic review. (See "HIV pre-exposure prophylaxis", section on 'Patient monitoring'.)

Presence of certain concurrent conditions — Some patients may have concurrent conditions that are impacted by the presence of HIV. As an example, individuals diagnosed with tuberculosis or latent tuberculosis infection should be tested for HIV, since HIV infection may lead to faster disease progression and important treatment considerations, including selection of anti-tuberculous medications to avoid interactions with antiretroviral agents. (See "Treatment of drug-susceptible pulmonary tuberculosis in nonpregnant adults with HIV infection: Initiation of therapy" and "Treatment of tuberculosis infection (latent tuberculosis) in nonpregnant adults with HIV infection".)

Patients diagnosed with chronic hepatitis B virus (HBV) infection should be tested for HIV given overlapping risk factors. In addition, patients with HIV/HBV coinfection require an antiretroviral treatment regimen that effectively suppresses both HIV and HBV (typically a three-drug regimen that includes tenofovir alafenamide or tenofovir disoproxil fumarate). (See "Treatment of chronic hepatitis B in patients with HIV".)

Similarly, individuals found to have hepatitis C virus (HCV) should also be screened for HIV due to overlapping risk factors, potential for accelerated progression of liver disease in the setting of HIV/HCV coinfection, and potential for drug-drug interactions between antiretroviral medications and HCV treatment options. (See "Treatment of chronic hepatitis C virus infection in the patient with HIV".)

ROUTINE SCREENING — All persons should be screened for HIV at least once in their lifetime. The frequency depends in part upon risk factors. The use of universal rather than targeted testing in ambulatory care settings can increase the number of new HIV diagnoses and reduce the proportion diagnosed late in the course of disease [3-7]. In addition, multiple studies have indicated that routine HIV testing is cost effective, especially in areas where the seroprevalence is ≥0.1 percent (the national average is 0.5 percent) [8-14]. (See 'Rationale for routine screening' below.)

Discussions related to consent and confidentiality are presented below. (See 'Consent and confidentiality' below.)

Persons with known risk factors for HIV — For persons with risk factors for HIV, routine screening should be performed regardless of age.

For those who are at high risk for HIV, we recommend annual or more frequent screening. This includes:

●Men and transgender women who have sex with men if they engage in condomless anal sex (insertive or receptive) with multiple or anonymous sex partners (or a main partner with HIV risk factors)

●Persons who inject drugs

●Persons who exchange sex for money or drugs

●Persons who have sex partners with uncontrolled HIV

●Persons from regions with generalized HIV epidemics who have condomless sex

We also suggest annual screening for other individuals who may be at risk for acquiring HIV due to the prevalence of HIV in their sexual network or a history of multiple sexual partners (eg, sexually active cisgender and transgender men who have sex with men [MSM] and transgender women who regularly use condoms or are in a monogamous relationship; heterosexual persons who themselves or whose sex partners have had >1 sex partner since their last HIV test). However, the risk in this group is lower than the group described above.

Patients with risk factors for HIV should be counseled regarding harm reduction, such as using clean needles if injecting drugs or using barrier protection for sex. (See "HIV infection: Risk factors and prevention strategies", section on 'Clinical approach to HIV prevention'.)

For individuals at ongoing risk for HIV, we discuss options for pre-exposure prophylaxis (PrEP) in addition to routine testing. (See "HIV pre-exposure prophylaxis".)

The Centers for Disease Control and Prevention (CDC) notes that sexually active MSM may benefit from testing at least annually and, in some instances, every three to six months [15,16]. We base the decision on whether to recommend screening once, annually, or more frequently based on an individual's personal risk factors, such as number of sex partners, whether HIV serostatus of partner(s) is known or unknown, frequency of condom usage, and history of other sexually transmitted infections. More frequent testing may be particularly important for MSM aged 13 to 24 years. In 2019, MSM in this age group accounted for 83 percent of all new HIV diagnoses; young Black/African American MSM were disproportionately affected, and this disparity persists due to structural racism, stigma, and other systemic factors [17,18].

Studies have also identified an increased frequency of HIV infection in transgender men and women, although data are more limited than for cisgender MSM. An analysis of data from 2006 to 2017 found an HIV prevalence of 14.1 percent for transgender women and 3.2 percent for transgender men in the United States [19]. Disparities by racial and ethnic background were evident, and the highest prevalence rate occurred among Black transgender individuals. Despite disproportionately high rates of HIV infection as compared with those of persons of other gender identities, transgender persons have relatively low rates of HIV testing. A 2014 to 2015 survey found that 37.5 percent of transgender women and 36.6 percent of transgender men ever had an HIV test, as compared with 66.2 percent of cisgender MSM [20]. Therefore, transgender women and men may benefit from counseling about the benefits of HIV testing and from routine testing, either annually or more often based on specific risk factors.

The importance of repeat testing among those at ongoing risk for HIV is supported by findings from the CDC National HIV Surveillance System [21]. Between 2006 and 2009, there were an estimated 125,104 new diagnoses of HIV. For those with a detailed testing history, 41 percent were diagnosed on their first test, while 59 percent were diagnosed with within one to two years after a negative test result.

Persons without known risk factors for HIV — We suggest at least one-time HIV screening for adults up to 65 years of age without known risk factors for HIV. This approach is consistent with guidelines from the US Preventive Services Task Force (USPSTF) [22]. The approach to routine screening in adolescents is presented in a separate topic review. (See "The adolescent with HIV infection", section on 'Routine screening'.)

In the United States, the CDC issued guidelines in 2006 advocating for routine, voluntary HIV screening for all patients aged 13 to 64 years as a normal part of medical care, without the need for signed consent or counseling [23]. Prior to that, risk-based screening had been the standard of care and signed consent had been required. The use of routine screening was introduced, in part, to reduce barriers to screening and to identify patients who may not disclose their risk factors or see themselves as at risk. However, risk assessment can be time consuming, and providers may not be able to reliably integrate risk assessment into routine care or may not be comfortable asking about HIV risk factors. As an example, in a study of 440 persons with HIV, of whom the majority had a CD4 count <350 cells/microL at the time of diagnosis, only 26 percent had HIV risk factors documented in the medical record more than one year before their diagnosis [24]. Furthermore, integrating HIV screening into routine medical care can help to standardize conversations about risk and normalize testing, which reduces stigma around these interventions.

The American College of Physicians (ACP) Guidance Statement on HIV screening, released in 2009, similarly endorsed universal screening, although it suggested that the age range be expanded to 75 years because of the growing number of HIV infections in older patients [25]. In 2013, the USPSTF recommended universal HIV screening among patients aged 15 to 65 years and all pregnant persons [26]. This recommendation was reaffirmed in 2019 [27].

Rationale for routine screening — Early detection of HIV, before a person is symptomatic, is important because it allows initiation of early antiretroviral therapy (ART), which reduces morbidity and mortality [28-31]. In the United States, individuals who are diagnosed early in the course of infection and successfully treated with ART may have life expectancies similar to those in the general population [32,33]. (See "When to initiate antiretroviral therapy in persons with HIV".)

Identifying individuals with HIV is also important because it reduces HIV transmission. Persons with HIV are unlikely to transmit HIV to others if they are receiving ART and have reductions in their viral load to <1000 copies/mL; in addition, being aware of one's HIV diagnosis can lead to changes in HIV risk behaviors [34,35]. (See "HIV infection: Risk factors and prevention strategies", section on 'Risk factors for infection'.)

Despite the importance of early diagnosis, a substantial number of individuals remain unaware that they have HIV [36-38]. In 2015, the World Health Organization (WHO) reported that only 54 percent of people with HIV knew their HIV status worldwide [36]. In the United States, an estimated 1.2 million persons were living with HIV in 2021, and nearly 13 percent had undiagnosed infection [1].

Based on 2021 data, the CDC found that the proportion of individuals with undiagnosed HIV varies by geographic region of the country, with the highest proportion of undiagnosed persons residing in the South [39,40]. There are also disparities in rates of undiagnosed HIV in the country by other demographic factors and social determinants of health. For example, one epidemiological study found that as of 2021 there were 153,500 individuals with undiagnosed HIV in the United States, and Black/African American and Hispanic/Latinx individuals were affected by disproportionately higher rates compared with White individuals [41].Between 2017 and 2021, nine percent of new HIV infections were identified in persons over 55 [42].

SCREENING DURING PREGNANCY — Pregnant persons should be tested for HIV early in each pregnancy using an "opt-out" approach, even if they have been screened during previous pregnancies. Repeat screening later in pregnancy may be indicated if risk factors are present, the person resides in a region with relatively high HIV incidence, or the person develops symptoms suggestive of HIV infection. (See "Prenatal care: Initial assessment", section on 'HIV'.)

TESTING ALGORITHM

Preferred approach — Our preferred testing algorithm uses a combination antigen/antibody HIV-1/2 immunoassay, followed by a confirmatory HIV-1/HIV-2 antibody differentiation immunoassay if the antigen/antibody screening test is positive (algorithm 1). This is in line with national testing recommendations from the Centers for Disease Control and Prevention (CDC) [43]. A discussion of the different types of HIV screening and diagnostic tests is found below. (See 'Overview of available tests' below.)

The combination antigen/antibody immunoassay detects both HIV type 1 (HIV-1) and HIV type 2 (HIV-2) antibodies, as well as HIV p24 antigen (table 2). Whenever possible, we use the combination tests designed for laboratory use, rather than the rapid lateral flow assay, because of increased sensitivity. (See 'Combination HIV antigen and antibody tests' below.)

●If the initial antigen/antibody test is negative – In most settings, if the combination antigen/antibody assay is negative, the person is considered not to have HIV, and no further testing is required.

However, additional testing should be performed if there are reasons to suspect the test may be a false negative (eg, concern for acute HIV or recent risk or exposure). (See 'False-negative or false-positive results' below.)

The approach to testing after an exposure or if there are signs and symptoms of acute HIV is discussed separately. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Diagnosis' and "Management of nonoccupational exposures to HIV and hepatitis B and C in adults" and "Management of health care personnel exposed to HIV" and "Management of nonoccupational exposures to HIV and hepatitis B and C in adults", section on 'Follow-up HIV testing (regardless of HIV PEP)'.)

●If the initial antigen/antibody test is positive – If the combination antigen/antibody assay is positive, a confirmatory HIV-1/HIV-2 antibody differentiation immunoassay is performed. In addition to confirming the diagnosis, the HIV-1/HIV-2 differentiation assay determines if a patient has HIV-1 infection or HIV-2 infection (or both). This distinction has important implications for HIV disease progression and treatment [44]. (See "Epidemiology, transmission, natural history, and pathogenesis of HIV-2 infection" and "Treatment of HIV-2 infection".)

If the antigen/antibody test is positive and the confirmatory HIV-1/HIV-2 antibody differentiation immunoassay is positive, the person has HIV and should be linked to care for appropriate management (see "Initial evaluation of adults with HIV"). False-positive initial and confirmatory testing rarely occurs. (See 'False-negative or false-positive results' below.)

If the antigen/antibody test is positive and the confirmatory HIV-1/HIV-2 antibody differentiation immunoassay is negative, this is considered an indeterminate test result, and additional evaluation is needed. (See 'Management of indeterminate test results' below.)

Our approach to HIV testing is based upon recommendations from the CDC [45]. Combination antigen/antibody tests are better able to identify acute/early infection compared with antibody-only tests (table 2) [46]. Using the CDC-recommended algorithm, an HIV screening program at a Phoenix, Arizona emergency department identified 37 undiagnosed HIV infections from July 2011 to February 2013. Of these, 12 (32.4 percent) were acute infections [47]. In another prospective evaluation of this algorithm (the STOP study), investigators diagnosed 610 previously undiagnosed HIV infections, of which 55 were acute [47]. Applying this new HIV testing algorithm averted missed diagnoses in 32 percent of the HIV-infected patients in the Phoenix emergency department and 9 percent of those in the STOP study.

The World Health Organization (WHO) has issued recommendations for HIV testing in low- and middle-income countries [48]. Detailed information can be found on the WHO website. All testing must be voluntary and confidential. Testing programs should also provide links to prevention, care, and treatment services.

Alternative approaches — Rapid, point-of-care antigen-antibody or antibody screening tests are sometimes performed for convenience and/or cost reasons [49]. Similar to laboratory based tests, antigen-antibody tests detect HIV earlier in the disease.

A positive rapid test is only preliminary. A laboratory-based combination antigen/antibody assay should be performed to confirm the results of the rapid test. An HIV-1/HIV-2 antibody differentiation assay is needed if the combination immunoassay is positive [45]. (See 'Overview of available tests' below.)

As with the preferred testing algorithm described above, HIV RNA testing (either qualitative or quantitative) should be performed if there is concern for acute HIV infection. A detailed discussion of how to diagnose acute HIV infection is found elsewhere. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Diagnosis'.)

False-negative or false-positive results — Although rare, false-negative and false-positive screening and/or confirmatory test results can occur. They can be a result of patient-related factors (eg, lack or low level of HIV antibodies, cross-reactive antibodies due to a concurrent condition) or the test itself (eg, low sensitivity, failure to detect certain subtypes of HIV, laboratory error during testing) [50-58].

●False-negative results – Most false-negative results are due to being in the "window period" after HIV acquisition, which is the phase before antigen or antibodies can be detected (ie, acute infection). False-negative screening results are more likely to be seen with antibody-only enzyme-linked immunosorbent assays (ELISA) tests than with combination antigen/antibody tests. The use of combination antigen/antibody assays has reduced (but not eliminated) the likelihood of a false-negative result by the addition of the p24 antigen test (table 2) [59,60]. Even the HIV RNA test can result in a negative very early after acquisition of HIV; it may take 7 to 21 days (mean 8 to 10 days) from date of infection for the HIV RNA level to become detectable by conventional assays (this is described as the "eclipse phase" of HIV) [61-64].

False-negative results have also been reported in very advanced HIV disease due to waning of the antibody response. Seroreversion can also occur when antiretroviral therapy (ART) is initiated very early during acute infection. In this situation, full antibody seroconversion may be delayed or may not occur, or initially positive antibody-based tests may revert to negative [65-68]. For this reason, we do not repeat antigen/antibody or antibody assays following initiation of ART. Should a patient with prior confirmation of HIV infection pursue repeat testing on their own and the result return as negative, they should be counseled not to stop their ART and should be informed of potential reasons for the negative result.

●False-positive results – Some patients may have a positive antigen/antibody test followed by a negative confirmatory test. This is considered an indeterminate test result and should typically be evaluated with an HIV RNA assay, as discussed below. (See 'Management of indeterminate test results' below.)

Many times, the work up for an indeterminate test will not indicate HIV infection [69]. Even though the specificity of HIV testing is high, when testing is done in a low prevalence community there will be a substantial number of false-positive results. This supports the need for confirmatory testing with an HIV RNA assay to rule in or rule out HIV. Additional information on false-positive HIV testing is available through the CDC.

A false-positive screening test (ie, positive antigen/antibody assay followed by negative differentiation assay and negative HIV RNA test) may result from factors not related to HIV infection, including:

•Cross-reacting alloantibodies from pregnancy

•Autoantibodies (associated with collagen-vascular diseases, autoimmune diseases, malignancy, certain classes of chemotherapy) [70]

•Receipt of an experimental HIV-1 vaccine [71]

•Influenza vaccination [72]

•Recent SARS-CoV-2 infection [73,74]

Often a false-positive screening result occurs, and no specific trigger or cause can be identified.

On rare occasion, both the screening and confirmatory tests are positive, and a subsequent HIV RNA level is very low or not detected. When this occurs, testing should be repeated. If the results persist, this could be consistent with the pattern of results seen in patients who are "HIV controllers" (persons with HIV and a low or undetectable HIV RNA in the absence of taking ART) or a person who is surreptitiously taking ART and has not disclosed this to the clinicians ordering the HIV test. (See "The natural history and clinical features of HIV infection in adults and adolescents", section on 'HIV controllers'.)

Management of indeterminate test results — Indeterminate test results occur when the initial screening test is positive (ie, ELISA or combination antigen/antibody assay) and the confirmatory test is indeterminate or negative, as discussed above. (See 'False-negative or false-positive results' above.)

We check plasma HIV RNA in all patients with an indeterminate test result. Repeating the initial testing is also reasonable, but an HIV RNA test should be performed to confirm the presence or absence of HIV infection (HIV RNA in this context refers to an HIV type 1 [HIV-1] RNA qualitative or quantitative test). For patients with a possible exposure to HIV-2, HIV-2-specific antibody and/or HIV-2 RNA testing should be performed. (See "Clinical manifestations and diagnosis of HIV-2 infection", section on 'Testing for HIV-2 infection'.)

In general, detectable HIV RNA is consistent with HIV infection and patients should be referred for appropriate care. Very rarely, a detectable HIV RNA at low levels (eg, <200 copies/mL) may represent a false-positive result due to the limitations of viral load testing. If this low-level result occurs, the viral load test should be repeated on a new blood specimen, or an alternative HIV RNA assay should be used to repeat the test. The diagnosis of HIV infection is confirmed if the second virologic test is positive or a serologic test done several weeks later is fully reactive.

If the HIV RNA test is negative, our approach depends upon the patient's risk for HIV infection:

●If the HIV RNA test is negative in a patient who is at low risk for HIV, they should be reassured that HIV infection is very unlikely. However, some providers repeat serologic testing approximately three months after the first test, as patients do not always disclose their risk factors. For most individuals, repeat testing remains indeterminate, often without a clear reason [75].

●For those with a recent high-risk exposure (eg, condomless anal sex, sharing needles) or with a high pretest probability of infection given past risk, we repeat an HIV RNA in one to two weeks. We repeat virologic testing sooner if the patient develops symptoms of acute infection. A detectable HIV RNA level confirms the presence of HIV infection and should prompt immediate referral for HIV care. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Clinical features'.)

●There are instances in which results of repeat serologic testing and HIV RNA testing remain ambiguous. In this setting, particularly if there are known risk factors for HIV or the individual is taking pre-exposure prophylaxis (PrEP), it may be useful to perform an HIV deoxyribonucleic acid (DNA) polymerase chain reaction (PCR) assay to assess for the presence of integrated proviral DNA. However, we do not check this type of assay routinely, and this should be done in consultation with a specialist. (See 'Nucleic acid tests' below.)

Patients should be counseled to avoid activities that could transmit HIV (eg, condomless sex, sharing needles, donating blood) until evaluation of the indeterminate test is complete. This is particularly true for individuals suspected of having acute HIV; given the elevated viral loads that are seen at this early stage of infection, these patients are at high risk of transmitting HIV to others. (See "Acute and early HIV infection: Pathogenesis and epidemiology", section on 'Infectivity'.)

For individuals taking oral or injectable HIV PrEP, indeterminate HIV test results on routine screening may be more common. The CDC recommends regular screening with an HIV RNA assay, especially for those receiving long-acting injectable PrEP. Patients receiving PrEP who have indeterminate or ambiguous test results should be referred to a specialist to help confirm the diagnosis. Clinicians can also contact the National Clinical Consultation Center, which offers free assistance interpreting and evaluating ambiguous HIV test results. (See "HIV pre-exposure prophylaxis", section on 'Patient monitoring'.)

OVERVIEW OF AVAILABLE TESTS — Available tests for the diagnosis of HIV primarily include antigen/antibody tests, antibody-only tests, and tests that detect RNA or DNA. The ability of these tests to detect HIV-1 and HIV-2, as well as certain groups of HIV (eg, M, N, O, and P), depends upon the type of test that is used.

The different tests for HIV are summarized in the table (table 2) and discussed below. Our suggested approach for screening and diagnosis of HIV infection (ie, testing algorithms) is presented above. (See 'Testing algorithm' above.)

Combination HIV antigen and antibody tests — Combination HIV antigen/antibody tests (previously referred to as "fourth generation") are distinguished from antibody-only tests by their ability to detect both HIV antibody and HIV p24 antigen [59,76-78]. Combined HIV antigen/antibody tests can detect HIV-1 and HIV-2, as well as group M and group O infections [52,53].

The sensitivity and specificity of combination antigen/antibody assays is reported to be greater than 99 percent [58,79-81]. In addition, these tests are able to identify acute/early infection in up to 80 percent of patients whose HIV diagnosis would have been missed by antibody-only testing, although they are not as sensitive as HIV RNA testing for acute infection [59].

Laboratory and point-of-care-based tests are available. Most laboratory tests are immunoassays. Point-of-care tests use lateral flow technology and are generally not as sensitive [78,82-85].

HIV antibody tests — (enzyme-linked immunosorbent assays [ELISAs], HIV-1/HIV-2 differentiation assays, Western blot) – HIV antibody tests can be used as a screening or confirmatory tests.

●Screening tests – Some ELISAs that are used for screening are performed in the laboratory, while others can be performed at the point of care. The sensitivity and specificity of laboratory-based tests approaches 100 percent for diagnosing chronic infection. Although these tests are not as sensitive as combination antigen/antibody tests for diagnosing acute infection, they are more sensitive than rapid antibody tests.

Rapid screening tests are designed to provide results in less than 20 minutes. Although these tests can be performed in the laboratory (using serum or plasma), some can also be performed in community-based settings supervised by trained personnel (called point-of-care tests), or at home (HIV self-tests), using whole blood or oral secretions.

The accuracy of most rapid tests is quite high (>99 percent sensitivity and specificity) for patients with chronic infection. However, in one study, rapid antibody tests missed approximately 12 percent of acute HIV infections [59]. In addition, testing on oral fluids appears to be less sensitive than testing on finger stick blood samples [86,87]. According to the Centers for Disease Control and Prevention (CDC), up to 8 percent of people with HIV can have a false-negative result when oral in-home tests are used [88].

●Confirmatory tests – The HIV-1/HIV-2 differentiation assay is a rapid laboratory-based test that is typically used to confirm a positive antigen/antibody combination assay and to distinguish between HIV-1 and HIV-2 infection. This test is used as part of the preferred testing algorithm described above (algorithm 1). (See 'Preferred approach' above.)

In the United States, the only US Food and Drug Administration (FDA)-approved differentiation assay that continues to be manufactured is the Bio-Rad Geenius HIV 1/2 confirmation assay [89]. With other tests, there was an increased risk of indeterminate results in low-risk populations [90,91].

Previously, an HIV-1 Western blot was used as a confirmatory test if a screening ELISA assay was positive. However, this test has several limitations. It can take up to two months after HIV acquisition for a Western blot assay to turn fully positive, and the test only reliably detects IgG antibodies to HIV-1, not HIV-2. In addition, Western Blot assays do not reliably detect subtype O virus [50,92]. For these reasons, most laboratories in the United States have ceased offering Western Blot testing.

Nucleic acid tests — Plasma HIV RNA or cell-associated HIV DNA can be measured using nucleic acid amplification techniques (termed NAT or NAAT). For HIV RNA testing, qualitative and quantitative assays are available. Quantitative HIV RNA tests are also referred to as viral load tests. (See "Techniques and interpretation of HIV-1 RNA quantitation".)

Routine screening of adolescents and adults for HIV with an RNA assay is generally not recommended, since it adds significant cost and, in general, does not increase the sensitivity of screening enough to make it cost effective.

However, the use of NAT as a diagnostic assay is indicated for:

●The evaluation of patients with an indeterminate serologic test (see 'Management of indeterminate test results' above)

●The evaluation of suspected acute infection in someone who may be in the "window period" of HIV seroconversion (see "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Diagnosis')

●Screening of blood donors (see "Blood donor screening: Laboratory testing", section on 'HIV-1 and HIV-2')

●Routine screening of individuals taking HIV pre-exposure prophylaxis (PrEP), particularly if injectable therapy is used (see "HIV pre-exposure prophylaxis", section on 'Patient monitoring')

Qualitative HIV RNA testing is primarily used as a screening test to identify individuals with HIV, such as possible blood donors. Although quantification of HIV RNA (viral load measurements) is designed primarily for use in the management and monitoring of individuals with HIV, this test is often utilized for diagnosis in patients suspected of having acute HIV infection, since most clinicians have more ready access to the quantitative tests. In addition, a baseline (pre-antiretroviral therapy) HIV RNA level is needed if a person is confirmed to have acute HIV infection. (See "Blood donor screening: Laboratory testing", section on 'HIV-1 and HIV-2' and "Techniques and interpretation of HIV-1 RNA quantitation" and "Patient monitoring during HIV antiretroviral therapy", section on 'Virologic response'.)

The HIV DNA NAT tests are used rarely as part of testing for HIV in adults. We reserve this test for situations in which the diagnosis remains unclear after the screening test, confirmatory test, and HIV RNA assay have been performed, such as for an individual who has risk for HIV, takes PrEP, and has ambiguous test results.

Point-of-care HIV NAT tests are in development but not yet commercially available [93].

ADDITIONAL CONSIDERATIONS

Consent and confidentiality — Screening should be voluntary and undertaken only with the patient's knowledge and understanding that HIV testing is being planned [23,36]. In most countries, a separate written consent for HIV testing is not required. In the United States, individual states have specific policies regarding the consenting process; however, no state still requires written consent by law. Information can be obtained from the Centers for Disease Control and Prevention (CDC) [94] and on the National Association of State and Territorial AIDS Directors (NASTAD) website.

In most settings, routine testing is implemented through a strategy called "opt-out testing," whereby the patient is informed orally or in writing that HIV testing will be performed [23]. The patient may then elect to decline or defer testing. If the patient opts out, this is then recorded in the medical record. The use of "opt-out" testing is routinely done in the United Kingdom, and one study reported an increased percentage of individuals being tested (from 35 to 65 percent) after this approach was introduced [95].

Implementation of routine HIV screening programs must be accompanied by scrupulous attention to patient confidentiality and appropriate counseling for post-test results [96]. Respect for patient confidentiality is necessary to minimize discrimination in insurance, employment, health care, and personal relationships, especially if someone tests positive for HIV.

Referral to care — If a patient is diagnosed with HIV, it is important to promptly refer and connect them to appropriate medical care. The CDC estimates that, during 2017, only 78 percent of individuals who received a diagnosis of HIV were linked to care within one month [97,98]. Prompt linkage to care can help ensure that all persons with HIV receive early antiretroviral therapy (ART) and appropriate primary and preventive care. Continued efforts are needed to improve all aspects of this treatment cascade. A number of strategies to improve linkage to care have been described [99].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: HIV screening and diagnostic testing".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: HIV and AIDS (The Basics)")

●Beyond the Basics topics (see "Patient education: Testing for HIV (Beyond the Basics)" and "Patient education: Symptoms of HIV (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●Importance of HIV testing and screening – Approximately 13 percent of persons with HIV in the United States remain unaware of their diagnosis. Improved access to HIV testing and improved testing algorithms can decrease the number of individuals who present with advanced immunocompromise and enhance the detection of newly-acquired infection. Patients who are treated with antiretroviral therapy (ART) before developing significant immunosuppression can expect a life expectancy approaching that of the general population. In addition, patients with very low viral loads on ART are unlikely to transmit HIV to others. (See 'Introduction' above.)

●Indications for testing – HIV testing should be performed in patients with clinical signs and symptoms of acute or chronic infection. (See 'Signs and symptoms of HIV' above.)

Testing should also be performed in those with a possible exposure to HIV; this includes recent exposures as well as exposures that may have occurred in the past (eg, history of other sexually transmitted infection, or history of hepatitis B virus [HBV] or hepatitis C virus [HCV], which have similar modes of transmission). (See 'Signs and symptoms of HIV' above and 'Possible exposure to HIV' above.)

Testing is also important for patients diagnosed with certain conditions that may be impacted by the presence of HIV (eg, tuberculosis, HBV). (See 'Presence of certain concurrent conditions' above.)

●Screening – The approach to screening depends in part on a person's risk for acquiring HIV. (See 'Routine screening' above.)

•For those at ongoing high risk for acquiring HIV, we recommend screening at least annually (Grade 1A). (See 'Persons with known risk factors for HIV' above.)

This includes:

-Men and transgender women who have sex with men if they engage in condomless anal sex (insertive or receptive) with multiple or anonymous sex partners (or a main partner with HIV risk factors)

-Persons who inject drugs

-Persons who exchange sex for money or drugs

-Persons who have sex partners with uncontrolled HIV

-Persons from regions with generalized HIV epidemics who have condomless sex

In some situations, screening as frequently as every two to four months may be indicated.

•For other patients who may be at increased risk, we also suggest annual screening (Grade 2C). This includes men who have sex with men (MSM) and transgender women who regularly use condoms or are in a monogamous relationship or heterosexual persons who themselves or whose sex partners have had >1 sex partner since their last HIV test. Although such individuals may be at risk for HIV (eg, due to a relatively high HIV prevalence in their sexual network or having multiple sexual partners), their risk is lower than the groups described above. (See 'Persons without known risk factors for HIV' above.)

•In addition, we suggest one-time screening for HIV in all adult patients at their first health encounter regardless of whether risk factors are present (Grade 2C). Individuals at very low risk may reasonably defer screening; patients who defer screening should be offered screening at follow-up visits. (See 'Persons without known risk factors for HIV' above.)

•For pregnant women, HIV testing should be performed during each pregnancy. Repeat screening later in pregnancy may be indicated if risk factors are present, the person resides in a region with relatively high HIV incidence, or the person develops symptoms suggestive of HIV infection. (See 'Screening during pregnancy' above and "Prenatal care: Initial assessment", section on 'HIV'.)

●Approach to testing – For routine HIV screening and diagnosis, we use a combination HIV-1/2 immunoassay that detects HIV p24 antigen and HIV antibodies (table 2); if positive, a confirmatory HIV-1/HIV-2 antibody differentiation immunoassay should be performed (algorithm 1). The combination antigen/antibody tests are better able to identify acute/early infection compared with antibody-only tests, since they can detect HIV p24 antigen at a time when antibodies may not yet be present. (See 'Preferred approach' above and 'Overview of available tests' above.)

If there is concern for acute HIV infection (eg, the patient presents with mononucleosis-like symptoms and had a recent possible exposure), additional testing with HIV RNA (either qualitative or quantitative) should be performed. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Diagnosis'.)

●Approach to indeterminate tests – Indeterminate test results occur when the initial screening test (eg, combination antigen/antibody assay) and the confirmatory test (eg, HIV-1/HIV-2 differentiation assay) are indeterminate or negative. An indeterminate test may result from recently-acquired HIV infection or may be the result of a false-positive screening test. Plasma HIV RNA should be tested in patients with an indeterminate test result.(See 'Management of indeterminate test results' above.)

We reserve HIV DNA nucleic acid amplification technique (termed NAT or NAAT) testing for atypical cases in which results remain ambiguous after the above evaluation, in the context of risk factors for HIV or clinical suspicion for HIV.