A borderline tuberculosis (TB) test result can be unsettling, leaving individuals unsure about their health status. Such an outcome indicates neither a clear positive nor a definitive negative. Understanding what a borderline result signifies and the typical next steps can help clarify this situation.
What a TB Test Measures
TB tests detect if a person’s immune system has reacted to the bacteria Mycobacterium tuberculosis, which causes tuberculosis. The two primary types of tests are the Tuberculin Skin Test (TST), also known as the PPD test, and Interferon-Gamma Release Assays (IGRAs), such as QuantiFERON-TB Gold Plus (QFT-Plus) and T-SPOT.TB. A positive result on either test generally suggests exposure to or infection with Mycobacterium tuberculosis. However, these tests primarily indicate a latent TB infection, where the bacteria are present but inactive, rather than active TB disease. A borderline result introduces uncertainty, suggesting a weak or ambiguous immune response that falls between the clear positive and negative thresholds.
Decoding a Borderline Result
A “borderline” TB test result indicates that the immune response measured falls into an indeterminate or equivocal range. For the PPD skin test, a borderline result might manifest as an induration (a firm, raised area) that does not meet the clear positive criteria but is not entirely negative. The interpretation of induration size, measured in millimeters, varies depending on an individual’s risk factors. For instance, an induration of 5 mm or more can be considered positive in individuals with compromised immune systems, while 10 mm or more may be positive for healthcare workers or recent immigrants from high-prevalence areas. In individuals with no known risk factors, an induration of 15 mm or more is typically considered positive.
For IGRA tests like QuantiFERON-TB Gold Plus, a borderline result often falls within a specific numerical range, such as 0.15 to less than 0.35 IU/mL, though some studies suggest a broader range like 0.20 to 0.99 IU/mL. These values represent a weak or uncertain interferon-gamma response to the TB antigens, making definitive interpretation challenging.
Reasons for Ambiguity
Several factors can contribute to a borderline TB test result. One common reason is prior vaccination with Bacillus Calmette-Guérin (BCG), a vaccine used in many countries to prevent severe forms of TB. The BCG vaccine can cause a false positive or borderline reaction on a PPD skin test, particularly if vaccinated after infancy or within 10 years of testing. While IGRAs are less affected by BCG vaccination, they can still sometimes yield ambiguous results in vaccinated individuals.
Exposure to non-tuberculous mycobacteria (NTM) can also lead to a borderline result. These environmental bacteria are related to Mycobacterium tuberculosis but do not cause TB disease; however, they can sometimes trigger a weak, cross-reactive immune response detectable by TB tests. Additionally, a very recent TB infection might result in a borderline test if the immune system has not yet mounted a strong, detectable response. Individuals with weakened immune systems due to certain medical conditions or medications may also produce a borderline result, as their bodies may not be able to mount a robust enough immune response for a clear positive, even if infected.
Navigating Next Steps
Receiving a borderline TB test result necessitates consultation with a healthcare provider for personalized interpretation and guidance. The initial step often involves a thorough clinical evaluation, including a review of symptoms such as persistent cough, fever, night sweats, or unexplained weight loss, as well as an assessment of medical history and risk factors for TB exposure. Healthcare providers may recommend repeating the same type of TB test or performing a different one to clarify the initial result.
For instance, if a PPD test was borderline, an IGRA might be advised, or vice versa, as these tests measure immune responses differently. If there is any suspicion of active TB disease, or if risk factors are high, additional diagnostic tests may be ordered. These can include a chest X-ray to look for lung abnormalities or sputum culture tests, which can detect the presence of Mycobacterium tuberculosis bacteria in lung secretions. In some situations, especially with low risk factors and no symptoms, a healthcare provider might suggest a period of watchful waiting with subsequent re-evaluation.