The Tuberculin Skin Test (TST), often known as the Mantoux test, is the primary method used to screen for exposure to Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). This test identifies individuals who have been infected but may not have active disease. Understanding the procedure and what a negative TST result looks like is important, as it generally indicates the person has not been infected with TB bacteria.
Administering the Tuberculin Skin Test (TST)
The TST is performed by injecting a standardized solution called purified protein derivative (PPD) just under the top layer of skin, a technique known as an intradermal injection. This injection is typically administered on the inner surface of the forearm, where the skin is thin and easily observed. The injection is given using a very short, thin needle and a small amount of fluid, usually 0.1 milliliters of PPD.
If administered correctly, the injection immediately creates a small, temporary blister-like bump called a wheal, typically six to ten millimeters in diameter. This technique ensures the PPD remains localized near the surface, allowing the immune system to react if prior TB infection exists. The patient must return to the healthcare provider for the reading of the test between 48 and 72 hours after the injection.
If the patient does not return within the 72-hour window, the test is considered invalid, and a new injection must be administered. The reading must be performed by a trained professional because the interpretation relies on a specific measurement, not just a visual check. The procedure is designed to elicit a delayed-type hypersensitivity reaction, which takes time to develop.
Visual Interpretation of a Negative Result
A negative TST result is determined by measuring the firm, raised swelling, or induration, at the injection site. Only the diameter of the induration is measured in millimeters, not any surrounding redness or bruising (erythema). Redness alone does not indicate a positive reaction and is often a sign of minor skin irritation from the injection itself.
For a result to be considered negative, there must be a lack of significant induration, which means the skin feels flat and soft when palpated. A measurement of zero millimeters of induration confirms a negative reading. In some cases, a very small bump, less than five millimeters, may also be classified as negative, depending on the person’s overall risk factors for TB exposure.
When palpating the injection site, a negative test will not present a hard, dense, or raised formation. The initial wheal disappears shortly after administration, and the skin returns to its normal appearance or may show only minor discoloration after two to three days. The reading requires gently sweeping the fingertips over the area to accurately locate any edges of induration.
Medical Significance of a Negative Result
A TST result with no or minimal induration indicates that the person has not been infected with Mycobacterium tuberculosis. This suggests the immune system has not developed a T-cell-mediated response to the PPD antigens following exposure. For most healthy, low-risk individuals, a negative test confirms that further testing or treatment for TB infection is unnecessary.
However, a negative result does not always rule out infection, a situation known as a false negative. This can occur if the person has a weakened immune system, such as those with HIV or those taking immunosuppressive drugs. A false negative can also happen if the TB infection is very recent, as the immune system requires between two and eight weeks to develop a detectable reaction to the PPD.
In cases of recent, high-risk exposure or active TB symptoms, a negative TST may require repeat testing or different diagnostic methods. Blood tests, such as interferon-gamma release assays (IGRAs), are often used as an alternative or follow-up to the TST because they are not affected by prior BCG vaccination. A negative TST is a favorable outcome, signifying the absence of a latent TB infection.