How to Interpret the Tuberculin Skin Test (TST) Result for Pulmonary Tuberculosis

Tuberculosis (TB) skin testing is a commonly used clinical method to determine whether a patient has been infected with Mycobacterium tuberculosis. Specifically, the purified protein derivative (PPD) test helps assess prior exposure to M. tuberculosis or prior Bacillus Calmette-Guérin (BCG) vaccination. Some patients yield positive results following PPD testing—so how should one interpret the results of a pulmonary TB skin test?

How to Interpret Pulmonary Tuberculosis Skin Test Results

The tuberculin skin test (TST) is an immunological assay that evaluates whether an individual’s immune system has developed hypersensitivity to M. tuberculosis antigens. In this test, tuberculin (PPD) is injected intradermally into the volar surface of the patient’s forearm. The reaction is assessed 72 hours later. Interpretation relies on measuring the diameter of induration (firm swelling), not erythema (redness). An induration measuring less than 5 mm in diameter is considered a negative reaction, typically indicating no prior infection with M. tuberculosis.

In unvaccinated children under three years of age, a positive result often suggests the presence of active pulmonary tuberculosis. A strongly positive reaction—regardless of age—indicates possible active pulmonary TB and warrants prompt identification of the primary lesion site and initiation of appropriate treatment.

In daily life, maintain a regular, balanced diet; avoid spicy, irritating, or overly rich foods; refrain from binge eating; and ensure adequate sleep—avoid staying up late. We hope this information is helpful to you.