PPD Test Result Interpretation Criteria
Nov 02, 2020
Source: Cainiu Health
Dr. Jiang Weimin
Introduction
PPD Test Interpretation Criteria:
A localized erythematous induration with a diameter < 5 mm is considered a negative reaction, typically indicating no prior infection with *Mycobacterium tuberculosis*.
A diameter ≥ 5 mm is considered a positive reaction; among these, reactions with a diameter ≥ 20 mm—or those < 20 mm but accompanied by local vesicles or necrosis—are classified as strongly positive.
The PPD test—also known as the tuberculin skin test (TST)—is an intradermal test that involves injecting tuberculin into the skin and evaluating the resulting cutaneous reaction at the injection site to determine whether a person has been infected with Mycobacterium tuberculosis. Clinical applications of the PPD test include: aiding in the diagnosis of tuberculosis (TB); assessing cellular immune response to the bacille Calmette–Guérin (BCG) vaccine three months after vaccination; and predicting prognosis in children with allergic constitutions.
Interpretation Criteria for PPD Test Results
PPD test results are typically assessed 72 hours after intradermal injection. Interpretation is based on the diameter of induration (firm swelling) at the injection site—not erythema (redness). A diameter of less than 5 mm is considered a negative reaction, generally indicating no prior infection with Mycobacterium tuberculosis. A diameter ≥5 mm is interpreted as a positive reaction. Among positive reactions, those with induration ≥20 mm—or those with induration <20 mm but accompanied by vesicles or necrosis—are classified as strongly positive.
Additional diagnostic tests are required to definitively rule out TB infection. The PPD skin test alone cannot confirm or exclude TB infection. Clinically, some patients with unequivocally diagnosed TB may yield negative PPD results—often due to compromised immunity, particularly among elderly individuals or those with severe concurrent infections. In such cases, the PPD test may fail to elicit a detectable immune response. Therefore, all diagnostic tests—including the PPD test—should be interpreted as supportive rather than definitive evidence.
Post-PPD Test Precautions
The PPD tuberculin test requires observation of the injection site at 72 hours post-administration. During this period, the injection site must remain dry to prevent secondary infection; swimming is strictly prohibited, as it may interfere with accurate interpretation of results. Patients should avoid spicy, greasy foods and alcohol consumption. The PPD test is an early auxiliary diagnostic tool for detecting TB infection. Individuals experiencing general malaise or systemic symptoms are advised to defer testing until their condition stabilizes.
The above outlines the criteria for interpreting PPD test results. We hope this information is helpful to you.
