What are the indicators of a false positive QuantiFERON (Interferon-Gamma Release Assay) test result?

From the Guidelines

A false positive QuantiFERON-TB Gold test can be identified through clinical correlation, consideration of risk factors, and confirmatory testing, as the test's positive predictive value is lower in low TB prevalence populations. To determine if a QuantiFERON-TB Gold test is false positive, several approaches can be taken. First, consider risk factors for false positives including prior BCG vaccination, infection with non-tuberculous mycobacteria, laboratory error, or improper handling of blood samples 1. Clinical correlation is essential - the absence of TB symptoms (cough, fever, night sweats, weight loss), normal chest X-ray findings, and no known TB exposure suggest a possible false positive.

Key Considerations

• Confirmatory testing is recommended, including a tuberculin skin test (TST) or repeating the QuantiFERON test 1.
• If results remain questionable, additional diagnostic tests like chest imaging or sputum cultures may be necessary.
• The test measures interferon-gamma release from T-cells in response to TB antigens, but cross-reactivity can occur.
• Healthcare providers should interpret results in the context of the patient's complete clinical picture, TB risk factors, and medical history before making treatment decisions 1.

Risk Factors and Test Characteristics

• False positives are more common in low TB prevalence populations where the positive predictive value of the test is lower.
• The specificity of QFT-G was reported to be 98.1% in a study of 216 BCG-vaccinated Japanese nursing students, and a sensitivity of 89.0% was reported in 118 patients with culture-confirmed TB 1.
• The test's performance can be affected by various factors, including the population being tested and the presence of immunocompromising conditions.

Recommendations

• Healthcare providers should use a combination of clinical judgment, risk factor assessment, and confirmatory testing to determine the likelihood of a false positive QuantiFERON-TB Gold test result.
• The most recent guidelines for using interferon gamma release assays, including the QuantiFERON-TB Gold test, should be consulted for specific recommendations on test interpretation and use 1.

From the Research

Identifying False Positive QuantiFERON Gold Test Results

To determine if a QuantiFERON Gold test result is false positive, consider the following factors:

• Repeat testing: Studies have shown that repeat testing can help identify false-positive results 2. In one study, 69% of patients with weakly positive results tested negative upon retesting within ten weeks 3.
• Clinical and radiological data: Consider the patient's clinical and radiological data to determine the likelihood of tuberculosis infection 4.
• Sarcoidosis: In patients with sarcoidosis, a false-positive culture result is possible, and a negative QuantiFERON Gold test result can help strengthen the suspicion of a false-positive culture result 4.
• Immunosuppression: Immunosuppressed patients are more likely to have false-negative results, but this does not directly relate to false-positive results 5, 6.
• Test reproducibility: The QuantiFERON Gold test has varying reproducibility, and weakly positive results may not be reliable 3.

Factors Associated with False-Positive Results

While there is limited direct evidence on factors associated with false-positive QuantiFERON Gold test results, the following may be relevant:

• Weakly positive test results: These may be more likely to be false positives, especially if the patient is not at high risk for tuberculosis 3, 2.
• Patient population: Certain patient populations, such as those with sarcoidosis or immunosuppression, may be more prone to false-positive or false-negative results 5, 4, 6.