Tuberculosis (TB) is a serious global health concern caused by the bacterium Mycobacterium tuberculosis. Diagnosing TB infection often relies on a blood test known as an Interferon-Gamma Release Assay (IGRA), which includes commercial tests like QuantiFERON-TB Gold and T-SPOT.TB. A positive IGRA result typically suggests a person has been infected with the TB bacteria. However, sometimes the test yields a “false positive,” indicating infection when the M. tuberculosis bacterium is not actually present. Understanding the reasons for this is important for correctly interpreting the result and determining further medical steps.
Understanding How the Blood Test Detects TB
The IGRA test indirectly detects TB infection by measuring the body’s immune response to the bacteria. This assay relies on cell-mediated immunity, specifically measuring the release of interferon-gamma (IFN-γ) from specialized T-cells. When the test is performed, a patient’s blood sample is mixed with synthetic proteins, or antigens, that mimic those found on M. tuberculosis.
The primary antigens used are early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10). If a person has been infected with M. tuberculosis, their T-cells will be “sensitized” to these proteins and release a measurable amount of IFN-γ in response. The presence of this immune signaling molecule above a certain threshold leads to a positive result, indicating exposure to the TB complex bacteria. These antigens are absent from all Bacille Calmette-Guérin (BCG) vaccine strains and most other mycobacteria, making the IGRA generally more specific than the older tuberculin skin test (TST).
When Other Infections Mimic TB
Despite the high specificity of the IGRA, the most common biological cause of a false positive is cross-reactivity with other bacteria. This occurs when T-cells, sensitized by an infection other than M. tuberculosis, mistakenly recognize the test antigens. Certain species of Non-Tuberculous Mycobacteria (NTM) possess genes that are similar to the ones encoding the ESAT-6 and CFP-10 proteins.
NTM are common in the environment (soil and water) and can cause infections, particularly in individuals with underlying lung conditions. The species most frequently documented to cause IGRA cross-reactivity include Mycobacterium kansasii, Mycobacterium marinum, and Mycobacterium szulgai. If a person has been infected with one of these NTM species, their T-cells react to the synthetic ESAT-6 and CFP-10, leading to a false indication of TB exposure. This cross-reaction is due to the genetic similarity between the NTM species and the M. tuberculosis complex.
IGRAs were specifically designed to minimize the effect of the BCG vaccine, which frequently causes false positives in the PPD skin test. Since the ESAT-6 and CFP-10 antigens are absent from the BCG vaccine strain, the vaccine should not trigger a positive IGRA result. In rare instances, a very remote BCG vaccination or multiple vaccinations combined with exposure to cross-reactive NTM may contribute to a borderline positive result. Recent live virus vaccinations, such as the measles or mumps vaccine, can also temporarily activate the immune system and potentially lead to a transient false positive.
Procedural and Handling Errors
Errors that occur during the collection, handling, or processing of the blood sample can also lead to an inaccurate positive result. The IGRA is a sensitive assay that requires specific, time-bound steps to ensure the viability and function of the patient’s T-cells. For example, the blood sample must be processed within a specific window, often within 8 to 16 hours of collection, because delayed transport can compromise the integrity of the white blood cells.
Any technical deviation from the manufacturer’s protocol can affect the assay’s performance, such as incorrect incubation temperatures or insufficient mixing of the blood with the antigen tubes. Laboratory errors in the analytic phase, including technical issues with the ELISA-based measurement or contamination of reagents, have also been identified as causes of false positives. Manufacturing quality control issues, such as problems with a specific lot number of antigen tubes, can occasionally contribute to elevated false positive rates.
What Happens After a Positive Result
A positive IGRA result indicates that the immune system has responded to the TB antigens, suggesting infection, but it does not differentiate between latent TB infection (LTBI) and active TB disease. When a positive result is received, especially in a person with low risk factors for TB, a medical evaluation is necessary to determine the true significance of the result. This clinical assessment includes reviewing the patient’s medical history, potential exposure events, and any symptoms of active disease, such as persistent cough, fever, or unexplained weight loss.
The next step in the diagnostic cascade is typically a chest X-ray to look for signs of active pulmonary TB disease. If the chest X-ray is abnormal or if the patient has symptoms, additional microbiological examinations are performed, such as collecting sputum samples to culture for M. tuberculosis bacteria. If the clinical picture and subsequent tests do not align with a TB infection, the positive IGRA result may be treated as a false positive, likely due to NTM exposure or a technical error.
If a false positive is suspected, particularly if the initial result was borderline or low-positive, the healthcare provider may recommend re-testing the patient after a period of time. Re-testing helps determine if the result is reproducible or if it was a temporary, non-specific immune reaction. Only after active TB disease has been definitively ruled out can the provider consider a diagnosis of LTBI and discuss appropriate preventative treatment options.