From the Guidelines
The values 0.50 and 0.59 are considered borderline positive results for tuberculosis (TB) testing, specifically when referring to interferon-gamma release assay (IGRA) tests like QuantiFERON-TB Gold, and thus, retesting is generally recommended within 4-6 weeks to determine if there is a true infection. These results fall into a gray zone that is neither clearly positive nor negative. For borderline results in this range, clinical evaluation including symptom assessment, chest X-ray, and risk factor analysis should be performed during the waiting period 1. The borderline results occur because the test measures immune response to TB antigens, and factors like recent exposure, immunosuppression, or technical variability can affect results.
Clinical Evaluation and Management
According to the guidelines, individuals should be asked about symptoms of TB before being tested for LTBI, and chest radiography can be performed if efforts are intended also for active TB case finding 1. Individuals with TB symptoms or any radiological abnormality should be investigated further for active TB and other conditions. The World Health Organization (WHO) recommends that either TST or IGRA can be used to test for LTBI in high-income and upper-middle-income countries with an estimated TB incidence of less than 100 per 100 000 1.
Preventive Treatment
If the repeat test remains borderline or becomes positive, especially in high-risk individuals, preventive TB treatment might be considered after active TB disease is ruled out. The standard preventive regimen typically includes isoniazid for 6-9 months, or shorter alternatives like 3-4 months of isoniazid plus rifampin, or 3 months of weekly isoniazid plus rifapentine. It is essential to weigh the risks and benefits of treatment, considering the individual's risk factors and medical history.
Key Considerations
- Retesting within 4-6 weeks is recommended for borderline results
- Clinical evaluation, including symptom assessment and chest X-ray, should be performed during the waiting period
- Preventive treatment may be considered for high-risk individuals with positive or borderline results after active TB disease is ruled out
- The choice of treatment regimen should be individualized based on the patient's risk factors and medical history 1.
From the Research
Borderline Positive Results for Tuberculosis (TB)
- The results of 0.50 and 0.59 are considered borderline positive for Tuberculosis (TB) based on the QuantiFERON-TB Gold Plus (QFT-Plus) test, as indicated in the study 2.
- However, the study 2 also mentions that the performance of the QFT-Plus test can be affected by various clinical contexts and patient hematologic profiles.
- Another study 3 evaluated the diagnostic power of Mycobacterium tuberculosis T-cell (T.SPOT®.TB) assays for active TB and found that the sensitivity and specificity of the test were 89.78% and 63.16%, respectively.
- The study 3 also determined an optimal diagnostic cutoff value of 13.5 spot-forming cells (SFCs) per 2.5 × 10^5 peripheral blood mononuclear cells (PBMCs) for the T-SPOT®.TB assay.
- A prospective Bayesian analysis 4 found that the T-SPOT.TB blood test had limited added value in diagnosing active TB, with a sensitivity of 74% and a positive predictive value of 56%.
- The studies 5 and 6 do not provide information on the interpretation of borderline positive results for TB.
Interpretation of Test Results
- The interpretation of test results for TB can be complex and may depend on various factors, including the type of test used, the clinical context, and the patient's hematologic profile 2.
- The studies suggest that a combination of clinical evaluation and laboratory testing is necessary for accurate diagnosis of TB 3, 4.
- The use of optimal cutoff values and consideration of patient hematologic profiles can help improve the performance of TB tests 2.