From the Guidelines
If there is suspicion for a false positive Interferon-Gamma Release Assay (IGRA) test, it is recommended to reassess the likelihood of M. tuberculosis infection and consider repeat testing on a case-by-case basis. In healthy persons with a low likelihood of M. tuberculosis infection and progression to active tuberculosis, a single positive IGRA result should not be taken as reliable evidence of infection, as the probability of a false-positive result is higher 1. Repeat testing, with either the initial test or a different test, may be considered to confirm the initial test results.
When considering repeat testing, it is essential to individualize the decision based on the quality and magnitude of the test result, the probability of infection, the risk for disease if infected, and the risk for a poor outcome if disease occurs 1. Factors such as laboratory error, cross-reactivity with non-tuberculous mycobacteria, recent vaccination, or immune system variations should be taken into account when interpreting IGRA results.
Key considerations for repeat testing include:
- Collecting a new blood sample at least 1-2 weeks after the initial test
- Ensuring proper collection technique with correct blood volumes in appropriate tubes
- Transporting the sample at the correct temperature (typically room temperature)
- Processing the sample within manufacturer-specified timeframes (usually 8-30 hours) Clinical correlation with symptoms, risk factors, and other diagnostic tests like chest X-rays should always accompany interpretation of IGRA results to ensure accurate diagnosis and appropriate management.
From the Research
Suspicion of False Positive Interferon-Gamma Release Assay (IGRA) Test
- If there is suspicion for a false positive IGRA test, the decision to redraw the test should be based on individual patient characteristics and medical history.
- There is no direct evidence to suggest that a false positive IGRA test should be redrawn, but several studies highlight the importance of considering patient characteristics and medical history when interpreting IGRA results 2, 3, 4, 5, 6.
- Factors such as age, HIV co-infection, non-Hispanic white race/ethnicity, and being tested with T-SPOT.TB have been associated with negative IGRA results, which may be relevant when evaluating the likelihood of a false positive result 2.
- A negative IGRA result may obviate the need for anti-TB therapy in individuals with a positive tuberculin skin test (TST) result, highlighting the importance of considering multiple diagnostic tests when evaluating TB infection 3.
- The clinical interpretation of IGRA results can be complex, and healthcare providers should consider the limitations and uncertainties of IGRA testing when making decisions about patient care 4.
- Patient characteristics such as increasing age, male sex, origin from a country with a high prevalence of tuberculosis, and increasing TST size have been associated with a positive IGRA result, which may be relevant when evaluating the likelihood of a false positive result 5.
- Host factors such as age, anti-tuberculosis treatment, HIV infection, connective tissue diseases, and low hemoglobin have been associated with false-negative IGRA results, highlighting the importance of considering individual patient characteristics when interpreting IGRA results 6.