How to Interpret the PPD Skin Test for Pulmonary Tuberculosis

Some individuals may develop this disease when their immune resistance is compromised—such as due to fatigue—or when they have coexisting illnesses. In other words, they may not be infected with *Mycobacterium tuberculosis*. Others may become ill immediately or during periods of low immunity, indicating prior infection with *M. tuberculosis* or previous Bacillus Calmette–Guérin (BCG) vaccination. So, how is the PPD skin test interpreted for pulmonary tuberculosis?

How to Interpret the PPD Skin Test for Pulmonary Tuberculosis

A PPD skin test result is considered positive if the induration diameter exceeds 0.5 cm, and strongly positive if it exceeds 2.0 cm. The larger the induration, the higher the likelihood of tuberculosis. However, a positive PPD test does not necessarily indicate active tuberculosis. Individuals who have received BCG vaccination may also test positive, as may those previously exposed to *M. tuberculosis*. Conversely, patients with negative PPD results may still be infected with *M. tuberculosis*, particularly those with impaired immunity—such as patients with malignancies, HIV infection, or those in the acute phase of measles.

Currently, the PPD test serves only as a clinical screening tool for tuberculosis—not as a definitive diagnostic criterion. Diagnosis requires comprehensive evaluation integrating imaging studies, sputum examination results, and other relevant tests.

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