TB is a serious global health concern caused by the bacterium Mycobacterium tuberculosis. The older tuberculin skin test was limited by cross-reactions with vaccines. Modern diagnostic technology introduced blood tests, known as Interferon Gamma Release Assays (IGRAs), which measure the immune system’s response to specific TB proteins. Although IGRAs represent a significant improvement in accuracy, they can occasionally produce a positive result in a person who has never been infected, known as a false positive. Understanding the factors that lead to these inaccurate results is important for proper diagnosis.
Immune System Recognition of Similar Bacteria
The most common biological reason for a false positive IGRA result is the immune system’s cross-reaction with bacteria other than M. tuberculosis. The IGRA test works by exposing a person’s T-cells to specific proteins, such as ESAT-6 and CFP-10, which are highly characteristic of the TB-causing organism. If the T-cells have previously encountered these proteins, they release a signaling molecule called interferon-gamma, which is then measured to indicate a positive result.
A group of organisms known as Non-Tuberculous Mycobacteria (NTMs) are common in the environment, often found in soil and water. Some NTM species, including Mycobacterium kansasii and Mycobacterium marinum, possess genetic sequences similar to those that code for the specific TB antigens used in the test. Exposure to these widespread environmental bacteria can sensitize T-cells, causing them to mistakenly recognize the test antigens and release interferon-gamma. This structural similarity in a few NTM species can still trigger a low-level positive reading, even if true TB infection is absent.
The Bacille Calmette–Guérin (BCG) vaccine is another factor. IGRAs are specifically engineered to exclude the antigens present in the BCG vaccine, making them generally unaffected by vaccination status. However, in cases of repeated BCG vaccination or recent immunization, a residual immune memory might still contribute to a positive result.
Errors in Sample Collection and Laboratory Processing
The accuracy of an IGRA test relies heavily on the correct handling of the blood sample, as the test measures a living immune response in real-time. Blood is drawn into specialized tubes containing the TB antigens and must be promptly incubated, often within 8 to 16 hours of collection, to keep the T-cells viable and reactive. Delays in transport or incubation can lead to the deterioration of the cells, which may result in an inaccurate reading.
The temperature during transport is also a consideration; the sample must be kept within a specific range, as extreme heat or cold can compromise the T-cell function. Improper technique during the initial blood draw, such as insufficient mixing of the blood with the antigens inside the collection tube, can prevent the immune reaction from occurring correctly. These pre-analytical errors can disrupt the delicate biological process required for the test, leading to a measured signal that does not accurately reflect the patient’s immune status.
Once the sample reaches the laboratory, technical mistakes can further introduce errors into the result. The incubation phase requires strict adherence to timing protocols to ensure an accurate measurement of the released interferon-gamma. Contamination of reagents, or issues with the laboratory equipment used to read the final result, such as the Enzyme-Linked Immunosorbent Assay (ELISA) reader, may also skew the final numerical concentration. A temporary malfunction or contamination within a specific batch of test components can mistakenly inflate the signal, pushing an otherwise negative result into the positive range.
Limitations in Test Specificity and Interpretation
Even with ideal biological and procedural conditions, the inherent characteristics of the IGRA test contribute to the potential for false positive results. The test relies on a statistical threshold, known as a cutoff, to categorize a result as either positive or negative. This cutoff represents an arbitrary line, and any result that falls just above this predetermined value is classified as positive, regardless of the underlying biological certainty.
This reliance on a cutoff can create a problem with borderline results, where the measured interferon-gamma level is only slightly higher than the threshold. Such low-level positive results are often biologically uncertain and may represent a transient or fluctuating immune response unrelated to true M. tuberculosis infection. Studies have shown that many individuals who test positive with a borderline value may revert to a negative result upon retesting, suggesting the initial positive was not a stable indicator of infection.
Another limitation stems from the test’s specificity, which, while high, is not absolute. The antigens ESAT-6 and CFP-10 are not perfectly unique to the M. tuberculosis complex, meaning that highly activated T-cells responding to other intense stimuli, such as recent live-virus vaccination or certain inflammatory conditions, could theoretically react.
Furthermore, slight differences in the manufacturing of test kits, known as inter-assay variability, can cause minor fluctuations in the measured signal. These small variations in the reagents or quality control between different batches can sometimes be enough to push a borderline negative result over the positive cutoff. This highlights the reliance of interpretation on statistical boundaries rather than a clear biological switch.