INTRODUCTION — Identification and treatment of individuals with latent tuberculosis (TB) infection (LTBI) is an important priority for TB control. The approach to diagnosis of LTBI is discussed separately. (See "Tuberculosis infection (latent tuberculosis) in adults: Approach to diagnosis (screening)".)
LTBI is a clinical diagnosis that is established by demonstrating prior TB infection and excluding active TB disease. Available tests to demonstrate prior TB infection include the tuberculin skin test (TST) and interferon-gamma release assays (IGRAs). These measure immune sensitization (type IV or delayed-type hypersensitivity) to mycobacterial protein antigens that might occur following exposure to (and infection by) mycobacteria (table 1). In order to minimize false-positive IGRA results, testing should be risk based; ordinarily, testing for TB infection should not performed on low-risk individuals.
There is no clear advantage of the IGRA or the TST for predicting future risk of active TB; the decision to select a test should be based on the setting, cost, and availability. Issues related to use and interpretation of IGRAs are reviewed here; issues related to interpretation of the TST are discussed separately. (See "Use of the tuberculin skin test for diagnosis of tuberculosis infection (tuberculosis screening) in adults".)
TERMINOLOGY — Tuberculosis terminology is inconsistent in the literature; relevant terms are defined in the table (table 2).
GENERAL PRINCIPLES — IGRAs are in vitro blood tests of cell-mediated immune response; they measure T cell release of interferon-gamma following stimulation by antigens unique to Mycobacterium tuberculosis and a few other mycobacteria . IGRAs are surrogate markers of M. tuberculosis infection and indicate a cellular immune response to M. tuberculosis protein antigens secreted by M. tuberculosis, but not by most non-tuberculous mycobacteria (NTM) or by Mycobacterium bovis Bacille Calmette-Guérin (BCG). In most individuals infected with M. tuberculosis, mononuclear cells in the blood release interferon-gamma when stimulated with antigens derived from M. tuberculosis.
To perform an IGRA test, a blood sample is incubated with antigens and controls . These tests require cells to remain viable and functional from the time of blood draw to completion of the incubation period with antigens. Preanalytical time and temperature constraints are critical to test performance and must be followed as specified by the manufacturer for each assay. IGRA results can be available in 24 to 48 hours (although it can take longer in routine practice because of batching of samples) and do not require a follow-up visit for reading of results (in contrast with the tuberculin skin test [TST]).
IGRAs cannot distinguish between latent infection and active TB disease and should not be used for diagnosis of active TB, which is a microbiologic diagnosis. A positive IGRA result may not necessarily indicate active TB, and a negative IGRA result may not rule out active TB [3,4]. (See "Diagnosis of pulmonary tuberculosis in adults".)
IGRAs are not affected by BCG vaccination status so are useful for evaluation of LTBI in BCG-vaccinated individuals, particularly in settings where BCG vaccination is administered after infancy or multiple (booster) BCG vaccinations are given. In contrast, the specificity of TST varies depending on timing of BCG and whether repeated (booster) vaccinations are given. A World Atlas of BCG Policies and Practices has been compiled to help clinicians and public health practitioners review variations in BCG policies and their impact on TST and to determine the populations for which IGRAs may be more specific than the TST .
IGRAs appear to be unaffected by most infections with environmental NTM, which can cause false-positive TSTs . In addition, limited data suggest that IGRA results are not affected by intravesical BCG for bladder cancer .
The antigens, testing methods, and interpretation criteria differ among IGRA assays. Some NTM that infect humans (including M. marinum, M. szulgai, M. flavescens, and M. kansasii) contain gene sequences that encode for antigens used in IGRAs (early secreted antigenic target 6 [ESAT-6] or culture filtrate protein 10 [CFP-10]). Infection with these NTM has been shown to produce positive results in IGRAs using these antigens (as with the TST) [8-10].
A large number of studies have evaluated IGRAs, and these have been summarized in systematic reviews and guidelines [2,4,11-22].
Assay antigens — A positive IGRA test may detect one or more antigens. M. tuberculosis-specific antigens include ESAT-6 and CFP-10. These are encoded by genes located within the region of difference 1 (RD1) segment of the M. tuberculosis genome. They are more specific for M. tuberculosis than purified protein derivative (PPD) because they are not shared with any BCG vaccine strains or most species of NTM (other than M. marinum, M. szulgai, M. flavescens, and M. kansasii) . The QFT-Gold In-Tube test also contains peptides from antigen 7.7, which is not encoded by these genes but are specific to M. tuberculosis. Antigen 7.7 is not included in the QFT-Gold In-Tube-Plus test (which has replaced the QFT-Gold In-Tube test in most settings).
Types of IGRA assays — Available IGRAs include QuantiFERON-TB assay and the T-SPOT.TB assay. Both tests are endorsed by the World Health Organization (WHO) , approved by the US Food and Drug Administration (FDA), and are also available in Canada, Europe, and many other countries. There has been considerable interest by in developing and marketing new IGRAs. In 2022, five new IGRAs or modifications of existing IGRAs were reviewed by an expert WHO panel; of these, only the Wantai-TB-IGRA was endorsed .
QuantiFERON — The QuantiFERON assay is an enzyme-linked immunosorbent assay (ELISA)-based whole-blood test that uses peptides from three TB antigens (ESAT-6, CFP-10, and TB7.7. As noted above, the antigen TB7.7 is not used in the QFT-Plus version).
Versions of the QuantiFERON assay include:
●QuantiFERON-TB Gold Plus (QFT-Plus) – Most recent assay; results almost identical to QFT-GIT; endorsed by the WHO 
●QuantiFERON-TB Gold In Tube (QFT-GIT) – Endorsed by the WHO
●QuantiFERON-TB Gold (QFT-G) – Earlier assay, no longer produced by the manufacturer
For the QFT-Plus test, blood is drawn into four tubes: two tubes TB antigens (TB1 and TB2), mitogen (positive control), or nothing (negative control). The result is reported as quantification of interferon-gamma in international units per mL. An individual is considered positive for M. tuberculosis infection if the interferon-gamma response to TB antigens is above the test cut-off (after subtracting the background interferon-gamma response in the negative control).
The added TB antigen tube in the QFT-Plus assay, compared with the older QFT-GIT assay (which has a single TB antigen tube), contains a mix of peptides, designed to measure CD8+ response, as discussed below. A positive result from either tube is interpreted as a positive test.
The same manufacturer has also produced a simplified test that uses lateral flow technology: the QIA Reach. This is designed to be a point-of-care test; however, thus far there are too few published studies evaluating the accuracy of this test . The manufacturer has also developed an automated high through-put method for analysis utilizing a chemiluminescent detection method, which replaces the ELISA technique for measurement of interferon gamma. Thus far, there are too few published studies evaluating the accuracy of this method.
Overall, the QFT-Plus appears to provide very similar results to the QFT-GIT; however, the performance of this test is still poorly defined in children and patients with human immunodeficiency virus (HIV) infection. There have been a few studies in these populations but without comparator tests (only QFT-Plus performed). In two studies, the sensitivity of QFT-Plus in HIV infected persons with active TB was 68.7 to 85 percent [24,25]. In three studies of children with active TB, the sensitivity of QFT-Plus was 83 to 84 percent [26-28].
Previous systematic reviews evaluated the sensitivity of IGRAs in patients with active TB reported a pooled sensitivity of 78 to 80 percent for QFT-G and 67 to 70 percent for QFT-GIT, similar to the pooled sensitivity of the TST (70 to 77 percent) but lower than the sensitivity of the T-SPOT.TB (90 to 93 percent) [12,13]. In the same reviews, the pooled specificity of QFT-G and QFT-GIT was greater than 95 percent among individuals considered to be at very low risk of LTBI. However, in these and other reviews, the sensitivity of QFT-G and QFT-GIT (as well as TST) was lower in people with HIV infection and children (particularly <5 years of age) [18,29-31].
Some studies have found a specific role of the CD8+ T cells in the immune response to M. tuberculosis infection in patients with and without HIV infection and in young children [32,33]. In an effort to improve the sensitivity of the QFT-GIT in young children, people with recent exposure, and people with HIV infection, the manufacturer developed the QFT-Plus which has one additional tube for induction of cell-mediated immune responses from both CD4+ and CD8+ T cells , with a positive test defined as either antigen tube positive. One review estimated that the QFT-Plus had sensitivity exceeding 90 percent in patients with active TB , considerably higher than the estimated sensitivity of QFT-GIT in earlier reviews. However, this estimate was based on studies in which participants underwent QFT Plus but no other comparator test. Since there may be important differences between the populations included in different studies, a less biased method is to calculate differences in sensitivity or specificity of QFT-Plus versus other tests only among individuals who underwent two or more tests simultaneously.
One review compared the sensitivity and specificity of QFT-Plus with QFT-GIT, T-SPOT.TB, and the TST, only in studies in which participants simultaneously underwent QFT-Plus and at least one of these other tests . Compared with QFT-GIT, the sensitivity of QFT-Plus in 7 studies including 661 patients with active TB was 1.3 percent higher (95% CI -0.3 to 2.9 percent); in 2 studies including 318 patients with very low probability of LTBI, the specificity was 0.9 percent lower (95% CI -2.4 to 0.6 percent) . Agreement between QFT-Plus and QFT-GIT was high; however, in 2 studies, within-person reproducibility of QFT-Plus was poor. All participants in the studies to estimate sensitivity were ≥15 years of age. The sensitivity of QFT-Plus and QFT-GIT was substantially higher than the sensitivity of QFT-GIT in all previous reviews [12,13]; this may reflect selection bias given limited data for children and patients with HIV infection.
Wantai TB-IGRA — The Wantai TB-IGRA is very similar in principle to the QuantiFERON-Gold-in-Tube. Each test kit is comprised of three tubes: Nil, mitogen, and TB antigens. The TB antigen tube contains a recombinant fusion protein of CFP-10 and ESAT-6. After incubation for 20 to 24 hours, the interferon gamma is detected by ELISA techniques.
The evidence reviewed to inform the WHO endorsement compared the sensitivity and specificity of Wantai TB-IGRA with QFT-GIT or QFT-Gold, or T-SPOT.TB, only in studies in which participants simultaneously underwent Wantai TB-IGRA and at least one of these other tests. In total, 22 eligible studies were included in the review; of these, 16 were judged at high risk of bias, largely because of missing methodologic information. All studies were conducted in China, and in all there was some involvement of the manufacturer. Further study is needed needed, given these limitations of the available evidence.
Compared with QFT-GIT, the sensitivity of Wantai TB-IGRA in five studies including 1600 patients with active TB was 3.0 percent higher (95% CI -0.2 to 6.2 percent); compared with T-Spot-TB, sensitivity of Wantai TB-IGRA in five studies including 1288 TB patients was comparable (95% CI -7.5 to 7.5 percent). Wantai TB-IGRA was 2.6 percent (95% CI -4.2 to -0.9) less specific than QFT-GIT and 10.3 percent (95% CI -17.2 to -3.4) less specific than T-Spot .
T-spot — The T-SPOT.TB is an enzyme-linked immunospot assay performed on separated and counted peripheral blood mononuclear cells (PBMCs), which include circulating monocytes and lymphocytes; it uses ESAT-6 and CFP-10 peptides. The result is reported as number of interferon-gamma-producing T cells (spot-forming cells in antigen wells minus negative control wells). An individual is considered positive for M. tuberculosis infection if the spot counts in the TB antigen wells exceed a specific threshold relative to the control wells. Unlike QuantiFERON assays, the T-spot assay includes a borderline category.
The manufacturer has produced two modifications to this test: T-Cell extend, intended to allow storage of the whole blood samples at room temperature for up to 72 hours without affecting test accuracy, and T-Cell Select, intended to allow automated separation of lymphocytes from whole blood. Thus far, there are insufficient published studies evaluating these modifications .
Sensitivity and specificity — IGRAs have specificity >95 percent for diagnosis of TB infection, since they are not affected by BCG vaccination or most nontuberculous mycobacteria [12,13]. Since there is no way to establish TB infection with certainty in individuals who do not have TB disease, sensitivity for these tests has been defined by testing patients with culture-confirmed TB. The sensitivity for T-SPOT.TB appears to be higher than for QFT-GIT or TST (approximately 90, 80, and 80 percent, respectively) . The higher sensitivity of T-SPOT.TB may be useful for evaluating individuals with immunosuppressive conditions. Specificity of the new QFT-Plus test, using short peptides, is not clear.
IGRA sensitivity is diminished by HIV infection . Lower CD4 counts have been associated with higher rates of indeterminate IGRA results; this is especially the case with QFT-GIT . T-SPOT appears to be less affected by immunosuppression than QFT-GIT, likely because the procedure requires that an adequate number of PBMCs are placed in each test well (even if the overall peripheral blood lymphocyte count is low) and the test does not correct for absolute T lymphocyte number . It is uncertain whether the new QFT-Plus test is more sensitive than the QFT-GIT.
There has been concern that IGRAs are not sensitive in very young children, and so are not recommended for use in children <2 years of age. In a 2023 systematic review including 575 high-risk children under age 2 (mostly case contacts or those born in tuberculosis endemic regions) with negative IGRA test results, none progressed to tuberculosis disease — including none of 70 children who were TST positive but IGRA negative . These findings suggest adequate IGRA for children <2 years of age.
IGRAs should not be used for diagnosis of active TB . The sensitivity and specificity are poor, particularly in high-incidence settings [3,4]. In addition, sensitivity is diminished due to the temporary anergy of the acute illness, and immune-based tests cannot distinguish between active disease and latent infection .
TST specificity may be reduced both by prior BCG vaccination and prior infection with nontuberculous mycobacteria, and depends on the cut-point for defining a positive test. The effect of BCG on TST is much greater if given after the first year of life (approximately 60 percent specificity in populations where BCG is routinely given after infancy) . (See "Use of the tuberculin skin test for diagnosis of tuberculosis infection (tuberculosis screening) in adults".)
SELECTING AN ASSAY — Issues related to choosing between IGRA and tuberculin skin test for diagnosis of LTBI are discussed separately. This continues to be an area of evolving recommendations as new evidence is published and because the commercially available IGRAs continue to be developed. (See "Tuberculosis infection (latent tuberculosis) in adults: Approach to diagnosis (screening)", section on 'Diagnostic approach'.)
INTERPRETING IGRA RESULTS
General principles — Results of IGRAs should be interpreted with other relevant clinical information such as age, Bacille Calmette-Guérin (BCG) vaccination status, history of epidemiologic exposure to active TB, and risk factors that increase the risk of progression to active disease.
In general, in healthy individuals with a low likelihood of M. tuberculosis infection and low likelihood of progression to active TB if infected, a single positive IGRA result should not be taken as reliable evidence of M. tuberculosis infection. Rather, repeat testing with the initial test or another test should be considered on a case-by-case basis. This caution also applies to a single positive tuberculin skin test (TST) in such individuals.
An online TST/IGRA algorithm has been developed to facilitate the interpretation of these tests . The algorithm provides information on positive predictive value of a test result along with predicted risk of progression to disease that may be used to help inform clinical judgment.
Reference range — The majority of IGRAs are reported as either positive, negative, borderline, indeterminate, or invalid based on the cutoff established by the test manufacturer, with numeric values (ie, absolute interferon [IFN]-gamma values for QuantiFERON-TB Gold [QFT-GIT] or number of spot forming cells for the T-SPOT TB test).
Reported results — In general, IGRA results are reported as one of the following:
●Uninterpretable results (may be reported as indeterminate, borderline or invalid, depending on the test manufacturer) (See 'Uninterpretable results' below.)
In addition, numeric values (ie, absolute interferon [IFN]-gamma values for QuantiFERON-TB Gold [QFT-GIT] or number of spot-forming cells for the T-SPOT TB test) are provided.
Antigens, testing methods, and interpretation criteria differ between IGRAs. Interpretation of an abnormal result should be based on the reference range reported for that result and the clinical context in which the test was applied. Borderline or indeterminate results indicate uncertain likelihood of M. tuberculosis infection.
Negative IGRA — Among patients with a negative IGRA, LTBI is less likely than in those with a positive IGRA. However, false-negative results can occur because of immunosuppression (eg, HIV or active TB, anergy, treatment with biologic agents), faded immune memory, and technical-operational variability. Among HIV-infected individuals, false-negative IGRA test results are more common with lower CD4 counts, particularly with the QFT tests . Treatment for LTBI despite a negative test can be considered on an individual basis. (See "Treatment of tuberculosis infection (latent tuberculosis) in nonpregnant adults without HIV infection" and "Treatment of tuberculosis infection (latent tuberculosis) in nonpregnant adults with HIV infection".)
Positive IGRA — Patients with a positive IGRA (in absence findings suggestive of active TB disease) should be considered for treatment of LTBI. The decision for treatment of LTBI is based on the likelihood of developing active TB if not treated balanced against the risk of adverse events if treated. (See "Treatment of tuberculosis infection (latent tuberculosis) in nonpregnant adults without HIV infection" and "Treatment of tuberculosis infection (latent tuberculosis) in nonpregnant adults with HIV infection" and "Tuberculosis disease (active tuberculosis) in pregnancy".)
Individuals with a positive IGRA should have prompt evaluation to exclude active TB. (See "Tuberculosis infection (latent tuberculosis) in adults: Approach to diagnosis (screening)", section on 'Excluding TB disease' and "Diagnosis of pulmonary tuberculosis in adults".)
The evaluation includes:
●Detailed history to determine possible exposure to active TB disease, and symptoms of active disease (eg, cough >2 weeks duration, fevers, night sweats, weight loss)
●Comprehensive physical examination to assess for signs of active disease
●Chest radiograph (usually normal in the setting of latent TB but in 15 percent of individuals with positive tests for TB infection may show dense nodules or scarring, particularly in the upper lung zones, which may be calcified, or calcified nonenlarged regional lymph nodes, or pleural thickening )
Findings suggestive of TB disease should be investigated for active TB using sputum for acid-fast bacilli smear and culture and nucleic acid amplification tests in addition to chest radiographs. (See "Diagnosis of pulmonary tuberculosis in adults".)
Concomitant infection with HIV is an important risk factor for progression from LTBI to active disease. Patients with HIV infection may have asymptomatic active TB, and those with advanced immunosuppression may have atypical presentations of TB disease (such as mediastinal lymphadenopathy, extrapulmonary disease, or subclinical infection with a normal chest radiograph). (See "Initial evaluation of adults with HIV", section on 'Tuberculosis'.)
For patients with a positive IGRA in the absence of findings suggestive of TB disease, a decision regarding LTBI treatment should be based on the likelihood of developing active TB and the risks of adverse events associated with LTBI treatment. In a systematic review and meta-analysis including 34 studies and 581,956 person-years of follow-up, higher levels of interferon-gamma were associated with increased risk of progression to tuberculosis disease. Compared with results of 0 IU/ml, the relative risk (RR) of progression to TB among those with interferon-gamma levels of 0.35 IU/ml was 1.6 times higher (95% CI: 1.3-2.1); with levels of 1.0 IU/ml the risk was 2.9 times higher (95% CI 2.0-3.9); with levels of 5 IU/ml the risk was 11.4 times higher (95% CI 6.6-16.4); and at 10 IU/ml was 19.0 times higher (95% CI 13.1-26.9) . (See "Treatment of tuberculosis infection (latent tuberculosis) in nonpregnant adults without HIV infection" and "Treatment of tuberculosis infection (latent tuberculosis) in nonpregnant adults with HIV infection" and "Tuberculosis disease (active tuberculosis) in pregnancy" and "Tuberculosis infection (latent tuberculosis) in children".)
Uninterpretable results — The two commercially available IGRA assays (QuantiFERON and T-SPOT.TB) differ in the way uninterpretable results are defined by the manufacturer.
●For the QuantiFERON assay (and the Wantai-TB-IGRA):
•Indeterminate – The result is reported as indeterminate when there is a strong response to the negative control (indicating a high level of spontaneous secretion of interferon-gamma in the absence of antigen), or a weak response to the positive control.
Indeterminate results are more common in persons with other conditions or illnesses; these include pregnancy (especially at the time of delivery) , advanced HIV infection (the likelihood of indeterminate results increases as CD4 count levels decrease in HIV-infected individuals, due to a reduced T cell response in the positive control) , and coronavirus disease 2019 (COVID-19; in one United States study including inpatients and outpatients, 65 percent had indeterminate results due to low mitogen response) .
An indeterminate IGRA result should be repeated to make sure there are no technical or laboratory flaws. If the repeat result is also indeterminate, then the clinician cannot rely on IGRA for clinical decision making, except to assume that the patient is probably anergic . Other tests, risk factors, and clinical information must be used instead.
●For the T-SPOT.TB assay:
•Invalid – The result is reported as invalid when there is a detectable reaction in the negative control or an insufficient reaction in the positive control. An invalid result can indicate a problem with laboratory processing or a patient condition.
•Borderline – Results where the higher of the spot counts is such that the (antigen plate minus Nil) spot count is five, six, or seven spots should be considered Borderline.
Repeat testing — Patients with uninterpretable IGRA results require repeat IGRA testing (algorithm 1). If the patient has an illness associated with immune suppression, the T-SPOT.TB test is preferable if available, since it has a lower rate of uninterpretable results in such patients.
●Repeat IGRA negative – Patients with a negative repeat IGRA result should be managed in the same way as patients with a negative initial IGRA. (See 'Negative IGRA' above.)
●Repeat IGRA positive – Patients with a positive repeat IGRA result should be managed in the same way as patients with a positive initial IGRA. (See 'Positive IGRA' above.)
●Repeat IGRA uninterpretable – Patients with two uninterpretable IGRA results warrant TST, with interpretation as follows:
•Negative TST – Patients with a negative TST should be managed in the same way as patients with a negative initial IGRA. (See 'Negative IGRA' above.)
•Positive TST – Patients with positive TST should be managed in the same way as person with a positive initial IGRA. (See 'Positive IGRA' above.)
Dual testing (with IGRA as well as TST) can be useful in certain circumstances. (See "Tuberculosis infection (latent tuberculosis) in adults: Approach to diagnosis (screening)", section on 'Role of dual testing'.)
Serial testing, conversion, and reversion
General principles — Serial LTBI testing is warranted for individuals with ongoing potential exposure (algorithm 2). This may include residents or employees of homeless shelters or correctional facilities, and health care workers (HCWs) in some circumstances . Those undergoing serial annual testing should also undergo testing after a known episode of exposure . (See "Tuberculosis infection (latent tuberculosis) in adults: Approach to diagnosis (screening)", section on 'Increased risk of new infection'.)
Given the difficulties of interpretation of serial IGRA testing, we favor use of TST for serial testing. Baseline two-step testing is not required for repeated IGRA testing (as for TST). (See "Use of the tuberculin skin test for diagnosis of tuberculosis infection (tuberculosis screening) in adults", section on 'Serial testing'.)
United States guidelines recommend use of either TST or IGRA for serial testing  but acknowledge that criteria for interpreting changes in IGRA results remain uncertain . Canadian guidelines recommend that only TSTs be used for serial testing . European guidelines state that there is insufficient evidence on the positive predictive value of IGRAs for serial testing, although, given the available evidence for the use of IGRAs, a two-step approach (ie, performing an IGRA whenever a TST result is positive) could increase the specificity (depending on the BCG vaccination status) . (See "Tuberculosis infection (latent tuberculosis) in adults: Approach to diagnosis (screening)", section on 'Guidelines vary by country'.)
Interpretation of serial IGRA tests is challenging due to unexpectedly high rates of conversions and reversions in low- and high-incidence settings [17,48-52]. A number of studies have raised concerns regarding the optimal approach to interpretation of serial IGRA results, especially since most HCWs in the United States (and other low-incidence countries) are at low risk of TB exposure:
●In one study including more than 9000 HCWs in the United States, the observed IGRA conversion rate of 4.4 percent was 11-fold higher than the historic TST conversion rate of 0.4 percent . More than 70 percent of HCWs with apparent IGRA conversion reverted to negative within six months. Reversion was most likely in HCWs whose IGRA conversion was associated with an absolute increase in interferon-gamma results of 0.2 to 0.7 international units/mL.
●In another study of United States HCWs at institutions with low rates of TB, IGRA conversions from negative to positive occurred in 6 to 8 percent of cases; of these, 80 percent of individuals had negative tests, indicating that most apparent IGRA conversions in this setting were falsely positive .
●A subsequent study among more than 16,000 HCWs in the United States noted lower T-SPOT conversion and reversion rates than earlier studies, although the mean reversion rate of nearly 18 percent suggests a persistent problem with use of IGRAs for serial testing .
●A serial IGRA study among more than 5300 adolescents in South Africa (a TB-endemic setting) demonstrated a 24 percent reversion rate from positive to negative at 2 years. The risk of reversion was inversely related to the magnitude of the IGRA response; the risk of progression to TB did not differ between persistent converters and reverters. The number of subsequent TB cases in the reverter group was too small to establish a difference in risk from the persistent converter group .
Hence, the optimal approach to interpretation of serial IGRA test results is uncertain [21,48,57,58].
The use of IGRAs for serial testing is complicated by lack of data on optimal cutoffs and uncertainty regarding the optimal approach to interpretation of conversions and reversions [2,14,17,48-51,54,59]. Use of simplistic definitions for conversions (ie, change from negative to positive result) may result in higher conversion rates than expected based on the known epidemiologic profile of a given population. IGRA reversions are more likely to occur among those with interferon-gamma values just above the diagnostic threshold and in those with a discordant profile (TST negative but IGRA positive) [2,14,17,48-51]. Therefore, for individuals with IGRA results that are borderline (T-spot) or close to threshold cutoff (eg, QuantiFERON results of 0.35 to 1.0 international units/mL), repeat testing is suggested, although it may be reasonable to repeat the IGRA tests in all individuals with apparent conversion.
Data on the association between IGRA conversion and subsequent disease risk are limited [60,61]. In one study from South Africa including individuals undergoing serial IGRA testing with QuantiFERON-TB Gold In-Tube assay, the incidence of TB was 10-fold higher among IGRA converters (interferon-gamma value <0.2 international units/mL at day 0 and >0.7 international units/mL at day 360) than non-converters (interferon-gamma value <0.2 international units/mL at days 0 and 360) . The study noted a range of interferon-gamma results associated with uncertainty in interpretation (0.2 to 0.7 international units/mL).
●Factors prior to analysis (ie, sample agitation, temperature during transport, time elapsed prior to incubation, duration of incubation) – The time elapsed prior to incubation can have an important effect on test results; in one study of QuantiFERON, delays in incubation of 6 or 12 hours were associated with positive-to-negative reversion rates of 19 and 22 percent, respectively . Tube agitation technique and the quantity of blood volume can also introduce variations in QuantiFERON results .
●Test reproducibility within a single laboratory – Substantial variability has been observed when assays are repeated on the samples from the same individual on the same day. In one QuantiFERON study including 1086 test results obtained from 543 subjects, the qualitative result (ie, positive versus negative) of the second test was discordant from the first test in 8 percent of patients .
●Test reproducibility in different laboratories – Results have varied considerably when two samples taken from the same person on the same day are tested in different laboratories .
●Variability in response of a given individual observed on different samples collected on different days – Among 48 HCWs who underwent monthly testing with QuantiFERON-TB Gold for one year, conversions or reversions were observed in 52 percent of participants .
●Boosting by tuberculin testing has been a concern; however, in a systematic review including five studies and 219 individuals, there was no significant boosting and only minimal variability .
These issues with reproducibility have led to suggestions defining a borderline range for IGRA results. A borderline range has been established for the T-SPOT assay (5 to 7 spots); 8 or more spots is considered positive. Despite published evidence that individuals with increases of 0.2 to 0.7 international units/mL are most likely to undergo spontaneous reversion to negative [54,69], the manufacturer of the QFT-Plus assay suggests a dichotomous cut point. Based on the same evidence, we consider QFT-Plus results within the range of 0.2 to 0.7 as borderline.
Until more definitive evidence and consensus emerges, laboratories should report both quantitative results and qualitative results, and providers should base their interpretation on the quantitative IGRA results. To reduce assay variability, laboratories should ensure standardization of preanalytical procedures such as tube agitation, time interval between specimen collection and incubation, and duration of incubation.
LIMITATIONS OF IGRAS — The reagent costs of IGRAs are substantially higher than costs of tuberculin skin test (TST). The IGRAs must be performed in certified laboratories, which requires adequate equipment, personnel who are trained and supervised, and a quality assurance program (table 1).
In addition, IGRAs require fresh blood samples; transportation delays and/or inappropriate specimen collection, transport, and sample preparation conditions can affect test performance. Blood specimens for the QFT assay must be placed in an incubator as soon as possible and within 16 hours of blood collection. For the standard T-SPOT.TB assay, blood must be processed within eight hours of collection. However, if the T-Cell Xtend reagent is used, whole blood can be stored overnight prior to processing in the T-SPOT.TB assay.
Test kits must be transported and stored in optimum conditions to prevent exposure to excessive heat. Strict quality assurance is necessary to detect unusual patterns in results (such as a spike in the number of indeterminate results due to low mitogen response), and it is important to run both positive and negative controls with each assay. Areas for additional research have been identified [2,12,70].
IGRAs may not be used for detection of latent tuberculosis in patients with leprosy, since the early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10) antigens are cross reactive between Mycobacterium leprae and M. tuberculosis .
Uninterpretable results — Issues related to uninterpretable results are discussed above. (See 'Uninterpretable results' above.)
Window period — Data on time frame for IGRA conversion are limited; available evidence suggests that most IGRA conversions occur within four to seven weeks after TB exposure. However, in some cases, conversion may be delayed longer than three months; agreement between TST and IGRA show better concordance after this window period [72,73].
These are important considerations for using IGRA to screen contacts of a TB patient. If IGRAs are used in contact investigations, a single IGRA should be performed at the end of the window period (six- to eight-weeks after last known TB exposure).
Monitoring therapeutic response — IGRAs should not be used to monitor response to therapy, given potential problems with reproducibility, conversions, and reversions [74-76]. Although some data suggest that a large percentage of active TB patients become IGRA negative by the end of TB therapy, other studies do not support this; some data suggest that TB patients can remain IGRA positive even years after TB treatment. One study noted that changes in IGRA results were not associated with smear and culture conversion to negative results, reinforcing the lack of utility for active TB treatment monitoring . In a clinical trial among contacts with LTBI, isoniazid therapy played no role in observed decreases in M. tuberculosis antigen-specific T cell responses over time .
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: Diagnosis and treatment of tuberculosis".)
●Approach to diagnosis LTBI − Identification and treatment of individuals with latent tuberculosis (TB) infection (LTBI) is an important priority for TB control. LTBI is a clinical diagnosis that is established by demonstrating prior TB infection and excluding active TB disease. The approach to diagnosis of LTBI is discussed separately. (See "Tuberculosis infection (latent tuberculosis) in adults: Approach to diagnosis (screening)".)
•TST − Issues related to use of TST are discussed separately. (See "Use of the tuberculin skin test for diagnosis of tuberculosis infection (tuberculosis screening) in adults".)
•IGRAs − IGRAs are diagnostic tools for LTBI. They are in vitro blood tests of cell-mediated immune response to Mycobacterium tuberculosis and measure T cell release of interferon-gamma following stimulation by antigens specific to M. tuberculosis. IGRAs cannot distinguish between latent infection and active TB disease and should not be used for diagnosis of active TB. A negative IGRA does not rule out active TB at any age. (See 'General principles' above.)
●Types of IGRA assays − There are three major IGRAs available: the two for adequate data are available regarding test performance are the QuantiFERON-TB Gold In-Tube Plus (QFT-GIT Plus) assay and the T-SPOT.TB assay. QFT-GIT Plus is an enzyme-linked immunosorbent assay-based whole blood test. T-SPOT.TB is an enzyme-linked immunospot performed on separated peripheral blood mononuclear cells (PBMCs). An additional IGRA, the Wantai-TB-IGRA, has been endorsed by the World Health Organization; this IGRA appears to have similar accuracy to QFT-GIT or QFT-Plus. However, until further independent high quality studies are available, we favor caution in adopting this test. (See 'Types of IGRA assays' above.).
●Sensitivity and specificity (See 'Types of IGRA assays' above.)
•The sensitivity for T-SPOT.TB appears to be higher than QFT-GIT or TST, likely because the testing platform ensures that an adequate number of PBMCs are available even in the presence of low lymphocyte cell counts in whole blood.
•The sensitivity and specificity of the QFT-Plus assay appears to be comparable with the preceding versions of the QuantiFERON test. However, there is insufficient evidence of the diagnostic accuracy of this test in patients with HIV infection or children.
●IGRAs preferred for patients with BCG vaccination − IGRAs are not affected by Bacille Calmette-Guérin (BCG) vaccination status and have specificity >95 percent for diagnosis of LTBI. Therefore, IGRAs are preferred for patients with history of BCG vaccination, especially if received in countries where BCG is given following infancy or are repeated. (See 'General principles' above.)
●IGRAs should not be used for serial testing − For serial testing in populations with ongoing TB exposure, data are insufficient for interpretation of IGRA conversions and reversions, and most studies suggest poor assay reproducibility. In these settings, performance of the TST by specifically trained personnel is the test of choice. (See 'Serial testing, conversion, and reversion' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Madhukar Pai, MD, PhD, who contributed to an earlier version of this topic review.
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