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Screening and diagnostic testing for HIV infection

Screening and diagnostic testing for HIV infection
Author:
Paul E Sax, MD
Section Editor:
Martin S Hirsch, MD
Deputy Editor:
Jennifer Mitty, MD, MPH
Literature review current through: Jan 2024.
This topic last updated: May 27, 2021.

INTRODUCTION — Nearly 15 percent of HIV-infected persons in the United States remain unaware of their HIV infection, leading to significant morbidity and the risk of further transmission to others. Improved access to HIV testing and new testing algorithms can decrease the number of individuals who present with advanced immunocompromise, enhance the detection of newly infected individuals, and reduce transmission to others.

This topic will address screening and diagnostic testing for HIV infection in adults in clinical care settings. Detailed information about screening for HIV in the blood supply, the diagnosis of HIV infection in children, and the diagnosis of HIV infections 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".)

WHOM TO TEST — 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. HIV testing should also be incorporated into routine screening of healthy individuals, including pregnant women.

Symptoms of HIV infection — All patients with signs and symptoms of acute or chronic HIV infection should be tested. For patients presenting with clinical manifestations of acute HIV infection, testing for HIV RNA may be needed. A detailed discussion of the clinical manifestations of HIV infection is found elsewhere. (See "The natural history and clinical features of HIV infection in adults and adolescents".)

Possible HIV exposure — Patients who present after a known high-risk exposure to HIV (eg, sexual or percutaneous) should be tested for HIV infection and assessed for post-exposure antiretroviral therapy (ART). 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 risk factors for HIV transmission and the use of post-exposure prophylaxis are found elsewhere. (See "HIV infection: Risk factors and prevention strategies", section on 'Risk factors for infection' and "Management of nonoccupational exposures to HIV and hepatitis B and C in adults" and "Management of health care personnel exposed to HIV".)

HIV testing is also indicated for the following groups who are at high risk for exposure to HIV:

Individuals who seek screening for sexually transmitted infections [1]. This provides an opportunity for risk reduction education as well as diagnosis of HIV infection. (See "Screening for sexually transmitted infections".)

Patients who are considering pre-exposure antiretroviral prophylaxis as an HIV prevention strategy. For such patients, HIV testing should be performed prior to starting pre-exposure prophylaxis and every three months thereafter. Pre-exposure prophylaxis is discussed in a separate topic review. (See "HIV pre-exposure prophylaxis".)

Routine screening — For patients without risk factors for HIV infection, we recommend at least one-time HIV screening in adults and adolescents 13 to 75 years of age. In addition, pregnant women should be tested for HIV early in each pregnancy using an "opt-out" approach, even if they have been screened during previous pregnancies. (See "Prenatal care: Initial assessment", section on 'HIV' and 'Consent' below.)

Although one-time testing at a routine medical clinic visit is reasonable for most patients, annual or more frequent testing is recommended for high-risk persons including:

Men who have sex with men (MSM) with sexual partners who are HIV-infected or have unknown serostatus. The Centers for Disease Control and Prevention (CDC) notes that sexually active MSM may benefit from testing every three to six months [2,3]. Frequent testing is particularly important for MSM aged 13 to 24 years, where the rate of new infections increased by 43 percent from 2003 to 2014 [4].

Injection-drug users.

Persons who exchange sex for money or drugs.

Sex partners of persons who are HIV-infected, bisexual, or inject drugs.

Persons who have sex with partners whose HIV status is unknown.

The importance of repeat testing among those at ongoing risk is supported by findings from the CDC National HIV Surveillance System [5]. Between 2006 and 2009, there were an estimated 125,104 new diagnoses of HIV infection. For those with a detailed testing history, 41 percent were diagnosed on their first test, while 59 percent were diagnosed with HIV infection within one to two years after a negative test result.

The use of routine HIV testing in resource-limited settings is discussed below. (See 'Resource-limited settings' below.)

Rationale for routine screening — Identifying patients infected with HIV is important because it allows:

The institution of early ART, which reduces clinical events and mortality in patients with HIV infection [6-9]. In the United States, patients who are diagnosed early in the course of infection and are successfully treated with ART may have life expectancies similar to those in the general population [10,11]. However, in 2008, 33 percent of patients were diagnosed late in the course of infection and developed AIDS within one year of their initial HIV diagnosis [12]. (See "When to initiate antiretroviral therapy in persons with HIV".)

Appropriate preventive care (eg, immunizations and prophylactic antibiotics) [13]. (See "Initial evaluation of adults with HIV".)

Reduced HIV transmission to others through reductions in viral load secondary to ART, and changes in HIV risk behaviors [14,15]. (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 are infected with HIV. In 2015, the World Health Organization (WHO) reported that only 54 percent of people with HIV infection knew their HIV status worldwide [16]. In the United States, an estimated 1.1 million persons were living with HIV in 2015, and nearly 15 percent had undiagnosed infections [17].

The percentage of persons with undiagnosed HIV infection varies by age and geographic area. As an example, in the United States, the highest percentage of undiagnosed infections was in persons aged 13 to 24 years (51 percent), followed by persons aged 25 to 34 years (28 percent). With regards to geographic area, the percentage of individuals with undiagnosed HIV infection at the end of 2012 was greater in Louisiana (approximately 23 percent) compared with <10 percent in Colorado, Connecticut, Delaware, Hawaii, and New York [18].

In a national survey, in which 28,462 individuals at high risk for acquiring HIV (MSM and persons who inject drugs) were anonymously interviewed and screened for HIV infection, approximately 13 percent tested positive for HIV, and 9 percent were unaware of their diagnosis [19]. Of those 335 individuals who were unaware of their diagnosis, approximately 70 percent had seen a clinician in the prior year, but less than half had been offered HIV testing.

In the United States, the CDC issued guidelines in 2006 advocating routine voluntary HIV screening of all patients aged 13 to 64 years as a normal part of medical care, without the need for signed consent or counseling [20]. Prior to that, risk-based screening had been the standard of care. The use of routine screening was introduced, in part, to identify patients who may not disclose their risk factors or see themselves at risk. In addition, risk assessment can be time consuming, and providers may not be able to reliably integrate risk assessment into routine care. As an example, in a study of 440 HIV-infected patients, 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 [21].

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 [22]. In 2013, the US Preventive Services Task Force (USPSTF) recommended universal HIV screening among patients aged 15 to 65 years and all pregnant persons [23]. This recommendation was reaffirmed in 2019 [24]. The USPSTF also recommends that HIV screening be offered to younger adolescents and older adults who are at increased risk for infection. This includes MSM and active injection drug users, as well as those who have acquired or requested testing for sexually transmitted infections, have anal intercourse without a condom, have vaginal intercourse without a condom with more than one partner whose HIV status is unknown, have transactional sex (exchanges sex for drugs or money), and/or have a sex partner who is living with HIV or is in a high-risk category [24].

Multiple studies have indicated that routine HIV testing is cost effective in the United States, especially if the seroprevalence is ≥0.1 percent (national average of 0.5 percent) [25-31]. In addition, the use of universal rather than targeted testing in ambulatory care settings appears to increase the number of new HIV diagnoses and reduce the proportion diagnosed late in the course of disease [32-36]. As examples:

A cluster randomized trial in the United Kingdom compared an educational intervention promoting opt-out rapid HIV testing versus standard of care in community-based clinics where the HIV prevalence was high (8 patients per 1000 adults) [33]. Over approximately two years, there were more patients diagnosed with HIV in the practices that received the intervention (32 versus 14). In addition, there was a nonsignificant increase in the mean CD4 count at the time of diagnosis (356 versus 270 cells/microL). However, three clinics in the intervention arm withdrew (one because of workload reasons).

A single-center cluster randomized trial compared universal and targeted HIV screening in an urban emergency department [32]. The estimated seroprevalence was 0.36 percent. When universal screening was performed, 1915 of 4692 patients (41 percent) consented and six new HIV infections were diagnosed. When targeted screening was performed, 1813 of 4880 (37 percent) had no testing indication; of the 3067 remaining patients, 47 percent consented and three patients were diagnosed with HIV.

TESTING ALGORITHM

Preferred approach — Our preferred testing algorithm uses a fourth-generation antigen/antibody combination HIV-1/2 immunoassay plus a confirmatory HIV-1/HIV-2 antibody differentiation immunoassay (algorithm 1). A discussion of the different types of tests is found below. (See 'Tests' below.)

For HIV screening of asymptomatic individuals or patients with signs and symptoms of chronic infection, a fourth-generation combination HIV-1/2 immunoassay is initially performed. This assay detects both HIV-1 and HIV-2 antibodies as well as HIV P24 antigen. Whenever possible, we use the combination tests designed only for laboratory use, rather than the rapid lateral flow assay, because of increased sensitivity. (See 'Combination HIV antigen and antibody tests' below.)

If the fourth-generation combination assay is negative, the person is considered HIV-uninfected, and no further testing needs to be done.

If the fourth-generation combination assay is positive, an HIV-1/HIV-2 antibody differentiation immunoassay is performed. This test confirms the results of the combination assay and also provides information as to whether the patient is infected with HIV-1, HIV-2, or both viruses.

A plasma HIV RNA level should be obtained to evaluate for acute infection if the fourth-generation test is positive and the confirmatory HIV-1/HIV-2 antibody differentiation immunoassay is indeterminate or negative. (See 'Indeterminate test results' below.)

If there is concern for acute HIV infection (eg, the patient presents with mononucleosis-like symptoms and had a recent high-risk exposure), additional testing with HIV RNA (either qualitative or quantitative) should be performed. 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'.)

Our approach to HIV testing is based upon the recommendations from the Centers for Disease Control and Prevention (CDC) [37]. The rationale for the use of this algorithm is as follows:

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 when antibody may not yet be present (eg, "window period of acute HIV infection") [38]. Early detection is important because of the high risk of transmission that precedes seroconversion and the potential opportunity to improve health outcomes with early antiretroviral therapy (ART) [39,40]. (See "Acute and early HIV infection: Treatment", section on 'Rationale for initiation of ART in early infection'.)

The HIV-1/HIV-2 differentiation assays determine if a patient is infected with HIV-1 or HIV-2 (or both). Although HIV-2 is less common than HIV-1 in the United States, accounting for less than 0.2 percent of cases, this distinction has important implications for HIV treatment [41]. (See "Treatment of HIV-2 infection".)

Using this 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 HIV infections [42]. Another prospective evaluation of this algorithm (the STOP study) diagnosed 610 previously undiagnosed HIV infections, of which 55 were consistent with acute infection. 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.

Testing algorithms used in resource-limited settings are described elsewhere in this topic. (See 'Resource-limited settings' below.)

Alternative approaches — Routine screening for HIV was and, in some laboratories, is still done by using a two-tiered approach via an initial enzyme-linked immunosorbent assay (ELISA) that only detects the presence of antibodies, followed by a confirmatory Western blot if the initial ELISA is positive. Although the accuracy of this testing algorithm for patients with chronic infection is extremely high (>99 percent sensitivity and specificity), it can fail to diagnose individuals who are early in the course of their infection when antibody has not fully developed (table 1). It can also misclassify HIV-2 infection, with approximately 60 percent of HIV-2 infections being misclassified as HIV-1 [43]. (See 'Antibody-only tests' below.)

Rapid point-of-care antibody screening tests are sometimes performed for convenience and/or cost [44]. An initial positive rapid antibody test is only preliminary. In this setting, a laboratory-based combination antigen/antibody assay should be performed if the rapid test is positive. Confirmatory testing (eg, HIV-1/HIV-2 antibody differentiation assay) is only needed if the combination immunoassay is positive [37]. (See 'Screening tests' below.)

As with the preferred testing algorithm described above, HIV RNA (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'.)

Interpretation of results — The results of serologic testing algorithms are reported as positive, negative, or indeterminate:

The criteria for a positive test are a positive combination assay or ELISA followed by a positive confirmatory assay.

A negative test is a negative screening combination assay or ELISA.

An indeterminate result is when the combination assay or ELISA is positive but the confirmatory test is indeterminate or negative.

Although rare, false negative and false positive 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) [45-53].

False-negative screening results are more likely to be seen with antibody-only ELISA tests than the combination antigen/antibody tests since most false-negative results are due to the window period after HIV acquisition before antibodies are detected (ie, acute infection). 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 1) [54,55]. (See 'Screening tests' below.)

The frequency of both the screening and confirmatory test yielding a false-positive test result is extremely rare, even in low-prevalence areas. As an example, the false-positive rate using a third-generation antibody test and a confirmatory Western blot ranges from 0.0004 to 0.0007 percent [56-58].

Indeterminate test results

Reasons for an indeterminate result — Indeterminate test results occur when the initial screening test is positive (ie, third-generation ELISA or fourth-generation combination assay) and the confirmatory test is indeterminate or negative.

The most important HIV-related cause of an indeterminate test result is recently acquired infection. As examples:

A positive combination antigen/antibody test and a negative HIV differentiation assay may occur if the combination assay detects only p24 antigen. The subsequent HIV differentiation assay would be negative since it detects only antibody. (See 'Combination HIV antigen and antibody tests' below and 'HIV-1/HIV-2 differentiation immunoassay' below.)

A positive screening test (third-generation ELISA test or the combination antigen/antibody test) and a negative Western blot can be seen in acute infection since both types of screening tests can detect antibody before the Western blot meets criteria for being positive (table 1). This scenario can also be seen in HIV-2 infection, since the screening assays test for HIV-1 and HIV-2, whereas the confirmatory Western blot only tests for HIV-1.

The predictive value of a reactive fourth-generation combination assay for acute infection after a negative confirmatory test is less than 10 percent in most situations because the prevalence of acute HIV is so low. As a result, in most clinical settings, a reactive fourth-generation test with a negative confirmatory test will not indicate HIV infection when further work-up is performed [59]. (See 'Approach to an indeterminate result' below.)

Reasons for a positive screening test resulting from factors not related to HIV infection include:

Cross-reacting alloantibodies from pregnancy

Autoantibodies (collagen-vascular diseases, autoimmune diseases, and malignancy)

Receipt of an experimental HIV-1 vaccine [60]

Influenza vaccination [61]

Approach to an indeterminate result — We check a plasma HIV RNA in all patients with an indeterminate test result. For patients with a possible exposure to HIV-2, HIV-2-specific antibody and/or HIV-2 viral load testing should be performed if a Western blot was used as the confirmatory test. (See "Clinical manifestations and diagnosis of HIV-2 infection", section on 'Testing for HIV-2 infection'.)

In general, a 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, <1000 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. 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 is negative, our approach depends upon the patient’s risk for HIV infection:

If the HIV RNA is negative in a patient who is at low risk for HIV, he or she should be reassured that HIV infection is very unlikely. However, some providers repeat serologic testing approximately three months after the first test since patients do not always disclose their actual risk status. For most individuals, repeat testing remains indeterminate, often without a clear reason [62].

For those with a recent high-risk exposure (eg, unprotected anal sex, sharing needles), we repeat an HIV RNA in one to two weeks. We repeat virologic testing sooner if the patient develops symptoms of acute infection. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'Clinical features'.)

Patients should be counseled to avoid activities that could transmit HIV (eg, unsafe sex, sharing needles, donating blood) until evaluation of the indeterminate test is complete. This is particularly true for individuals suspected of having acute HIV infection; 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'.)

TESTS — Available tests for the diagnosis of HIV infection include those that can detect:

HIV antibody (enzyme-linked immunosorbent assays [ELISAs], HIV -1/HIV-2 differentiation assays, Western blot)

HIV antibody and HIV antigen

HIV antigen

HIV RNA (qualitative or quantitative)

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 discussed in this section. Our suggested approach for screening/diagnosis of HIV infection (ie, testing algorithm) is presented above. (See 'Testing algorithm' above.)

Screening tests

Antibody-only tests — ELISAs that detect antibody to HIV are used as an initial test to screen for HIV infection. These tests (referred to as a third-generation antibody test when detecting immunoglobulin M [IgM] as well as IgG antibodies) detect the presence of HIV-1 and HIV-2 antibody as early as three weeks after exposure to the virus (table 1). The ELISA antibody tests in the United States and Europe detect all M subtypes but do not consistently detect other groups [45,46,63,64].

Laboratory-based tests – Certain ELISA tests are only designed to be performed in the laboratory. While some provide results in less than an hour, others require a longer turnaround time (>3 hours) [65]. The sensitivity and specificity of these 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. A positive laboratory-based test should be confirmed with either an HIV-1/HIV-2 differentiation assay or, if unavailable, an HIV Western blot. (See 'HIV-1/HIV-2 differentiation immunoassay' below and 'Western blot' below and 'Combination HIV antigen and antibody tests' below.)

Rapid 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), they can also be performed in community-based settings supervised by trained personnel, or at home, using whole blood or oral secretions. Rapid tests have the advantage of offering preliminary test results at the same patient encounter, and are they well suited for patients who obtain sporadic medical care and for patients who are unlikely to return for their results [66].

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 [54]. In addition, testing on oral fluids appears to be less sensitive than testing on finger stick blood samples [67]. In a retrospective observational study of the use of oral fluid testing in detecting HIV seroconversion, 233 false-negative results occurred in 80 of 287 individuals [68]. According to the Centers for Disease Control and Prevention (CDC), up to 8 percent of HIV-infected people can have a false-negative result when oral in-home tests are used [69].

Combination HIV antigen and antibody tests — Fourth-generation HIV tests are distinguished from antibody-only tests by their ability to detect both HIV antibody and HIV p24 antigen [70-72]. Combined HIV antigen/antibody tests can detect HIV-1 and HIV-2 as well as group M and group O infections [47,48].

The sensitivity and specificity of the fourth-generation tests approach 100 percent for patients with chronic HIV infection. 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. As an example, in a prospective study of 86,836 patients recruited from 12 sexually transmitted infection and community-based clinics in the United States, those who tested negative for HIV using a rapid antibody test were then screened for HIV infection with a fourth-generation laboratory-based combination HIV assay and pooled HIV RNA testing [54]. Among the 85,690 patients who had a negative rapid antibody test, 168 patients were found to have acute HIV infection; the combination antibody and antigen test detected 134 (82 percent) of those that were detected by HIV RNA testing.

Three of the available combination tests in the United States are designed to provide rapid results (ie, within 30 minutes):

The ARCHITECT HIV Ag/Ab Combo test is an immunoassay that is designed to be performed in the laboratory. It can be used to analyze batches of specimens or to provide rapid results on a single sample.

The Elecsys HIV Duo test is another immunoassay designed to be performed in the laboratory.

The Determine HIV 1/2 Ag/Ab Combo provides rapid results using lateral flow technology on a single specimen.

Although the Determine HIV 1/2 Ag/Ab Combo test is well suited for point-of-care settings, laboratory-based combination tests that use an immunoassay are more sensitive and are better for diagnosing acute infection [72-75]. Studies that illustrate these findings include:

In an analysis of 21,234 consecutive venous blood samples obtained by San Francisco HIV testing programs from 2003 to 2008, the ARCHITECT HIV Ag/Ab Combo fourth-generation immunoassay identified 87 percent of acute HIV infections, whereas the Determine HIV1/2 Ag-Ab Combo test, using lateral flow technology, detected 54 percent of acute infections [76]. However, both combination assays detected more acute infections compared with antibody-only tests, which only detected 26 to 35 percent.

Another study evaluated the ability of the Determine Combo test to detect acute and/or early HIV-1 infection compared with other HIV tests [73]. The Determine Combo test was reactive approximately 15 to 16 days before a Western blot was positive and about two days before antibody-only tests were positive. However, the Determine Combo test was positive three to four days after a test that used technology similar to the ARCHITECT Combo test.

Confirmatory tests

HIV-1/HIV-2 differentiation immunoassay — The HIV-1/HIV-2 differentiation assay is a rapid laboratory-based test that is typically used to confirm a positive fourth-generation combination assay and to distinguish between HIV-1 and HIV-2 infection. The turnaround time for this confirmatory test is generally <20 minutes. This test is used as part of the preferred testing algorithm described above (algorithm 1). (See 'Preferred approach' above.)

Available HIV-1/HIV-2 differentiation tests include:

The Bio-Rad Geenius HIV1/2 confirmation assay

The Bio-Rad Multispot HIV-1/HIV-2

Both the Geenius and Multispot tests have high sensitivity for established infection (99.3 to 100 percent) [77,78]. The Multispot test is reported to have a specificity of 99.9 percent, whereas the Geenius test has a 4.3 percent indeterminate rate for low-risk populations [77].

Western blot — An HIV-1/HIV-2 differentiation assay is now the preferred confirmatory test. However, for many years, a confirmatory HIV-1 Western blot test was performed if an ELISA was positive to exclude the possibility of a false-positive screening test. The Western blot results usually took several days to weeks to return, as some clinic and hospital labs referred blood specimens for Western blot testing to reference labs. (See 'Alternative approaches' above.)

The Western blot detects IgG antibody to HIV-1; however, it can take up to two months after HIV acquisition for the test to turn fully positive. In addition, HIV-1 Western blot assays do not reliably detect subtype O virus [45,63], and if HIV-2 is being considered, a special HIV-2 Western blot must be requested. (See "Clinical manifestations and diagnosis of HIV-2 infection", section on 'Diagnostic issues related to HIV-1 Western blot testing'.)

Viral detection — Viral detection is an alternative approach to establishing an HIV diagnosis since virus is present in blood samples before HIV antibodies can be detected (table 1). The most commonly used methods detect HIV RNA or HIV p24 antigen. The P subgroup of HIV can only be detected with nucleotide sequencing [49]

Testing for plasma HIV RNA is not considered superior to routine serologic testing in most situations. However, the use of viral detection as a diagnostic assay is indicated for:

The diagnosis of neonatal HIV infection (see "Diagnostic testing for HIV infection in infants and children younger than 18 months")

The evaluation of patients with an indeterminate serologic test (see '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')

Plasma HIV RNA can be measured using qualitative or quantitative techniques. Qualitative testing (commonly referred to as nucleic acid testing [NAT]) is primarily used as a screening test to identify HIV-infected individuals, such as possible blood donors. Although quantification of HIV-RNA (viral load measurements) is designed primarily for use in the management and monitoring of HIV-1 infected individuals, this test is frequently utilized as a diagnostic test in suspected acute HIV infection since most clinicians have more ready access to quantitative viral load tests than NAT. (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'.)

Detection of HIV p24 antigen is another method for detecting virus. The p24 antigen usually becomes detectable at about one to two weeks after viral transmission [79]. This test is less expensive than nucleic acid sequence-based testing and can identify approximately 80 to 90 percent of individuals who seek testing during acute infection [80,81]. Although current p24 antigen tests are more sensitive than early generations of this assay, they have predominantly been replaced by HIV RNA testing. The most common use of p24 antigen testing now is as part of the fourth-generation combined antibody/antigen test. (See "Acute and early HIV infection: Clinical manifestations and diagnosis", section on 'HIV antigen detection'.)

PATIENT-RELATED ISSUES

Consent — Screening should be voluntary and undertaken only with the patient's knowledge and understanding that HIV testing is being planned [16,20]. 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 at the Centers for Disease Control and Prevention (CDC) [82] and on the National Association of State and Territorial AIDS Directors (NASTAD) website.

In some 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 [20]. 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 [83].

Counseling — Patients should be counseled regarding harm reduction, such as clean needle use by injection drug users or the use of barrier protection in those who have multiple sex partners. Persons with an indeterminate or positive HIV screening test need to be counseled that additional testing is required; however, in the interim, precautions should be taken to decrease transmission to others. An overview of HIV prevention strategies is discussed elsewhere. (See "HIV infection: Risk factors and prevention strategies", section on 'Clinical approach to HIV prevention'.)

Confidentiality — Institution of routine HIV screening programs must be accompanied by scrupulous attention to patient confidentiality and appropriate counseling for post-test results [84]. 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 infection, it is important to refer the patient for appropriate medical care. The CDC estimates that only 30 percent of the more than one million individuals in the United States who are living with HIV/AIDS are getting the full benefits of the treatment they need to manage their disease and keep the virus under control [85]. This estimate is based on surveillance data from 2011 that estimates for every 100 individuals living with HIV in the United States, 86 are aware of their HIV status, 40 are engaged in HIV care, 37 are prescribed antiretroviral therapy (ART), and 30 are able to adhere to their treatment and sustain undetectable viral loads.

One modelling study suggests strategies that combine enhanced testing and linkage with improved retention would reduce the HIV incidence by 54 percent and the mortality rate by 64 percent [86]. In San Francisco, a program that initiated ART as soon as possible after diagnosis was evaluated in a cohort of 39 patients with acute or recent infection (<6 months) or a CD4 count <200 cells/microL [87]. Approximately 95 percent initiated ART within 24 hours of diagnosis, and almost all received a regimen that included an integrase strand transfer inhibitor. The median time to achieve an HIV RNA <200 copies/mL was significantly faster among those who received the intervention compared with a cohort of 116 patients who received standard of care (1.8 versus 4.3 months). However, loss to follow-up was similar between the groups. Continued efforts are needed to improve all aspects of this treatment cascade.

RESOURCE-LIMITED SETTINGS — The World Health Organization (WHO) has issued recommendations for HIV testing in low- and middle-income countries [88]. Detailed information can be found on the WHO website. All testing must be voluntary and confidential. HIV testing efforts should also provide links to prevention, care, and treatment services. (See 'Patient-related issues' above.)

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 5th to 6th 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 10th to 12th 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/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

Nearly 15 percent of HIV-infected persons in the United States remain unaware of their HIV infection. Improved access to HIV testing and new testing algorithms can decrease the number of individuals who present with advanced immunocompromise, enhance the detection of newly infected individuals, and reduce transmission to others. (See 'Introduction' above.)

HIV testing should be performed to diagnose HIV infection in patients with clinical signs and symptoms of acute or chronic infection, as well as those with a possible exposure to HIV. (See 'Symptoms of HIV infection' above and 'Possible HIV exposure' above.)

For patients without identifiable risk factors for HIV acquisition, we recommend one-time HIV screening in adults and adolescents 13 to 75 years of age rather than risk-based screening (Grade 1B). HIV testing should also be performed for all pregnant women, even if they have been screened during previous pregnancies. (See 'Routine screening' above and 'Rationale for routine screening' above.)

For those with ongoing high-risk behaviors (eg, men who have sex with men with sexual partners who are HIV-infected or have unknown serostatus, injection-drug users, persons who exchange sex for money or drugs, sex partners of persons who are HIV-infected) annual or more frequent screening should be performed. (See 'Routine screening' above.)

For routine HIV screening and diagnosis, we use a fourth-generation combination HIV-1/2 immunoassay that detects HIV p24 antigen and HIV antibodies; 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 antibody may not yet be present. (See 'Preferred approach' above and 'Tests' above.)

If there is concern for acute HIV infection (eg, the patient presents with mononucleosis-like symptoms and had a recent high-risk 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'.)

Indeterminate test results occur when the initial screening test is positive (ie, third-generation antibody assay or fourth-generation combination assay) and the confirmatory test (HIV-1/HIV-2 differentiation assay or Western blot) is indeterminate or negative. An indeterminate test may result from recently acquired HIV infection or be the result of a false positive screening test. Plasma HIV RNA should be tested in all patients with an indeterminate test result. (See 'Indeterminate test results' above.)

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Topic 3736 Version 41.0

References

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