How to interpret PPD test results
Dec 03, 2025
Source: Cainiu Health
Dr. Ren Yi
Introduction
The results of the PPD test are generally analyzed based on dimensions such as the diameter of induration, presence of blistering or necrosis, grading of positive reaction intensity, and criteria for negative result determination. The diameter of skin induration, rather than erythema, should be measured. An induration diameter less than 5 mm is considered negative; 5 to 9 mm is weakly positive; 10 to 19 mm is moderately positive; and ≥20 mm, or the presence of blistering or necrosis at the injection site, is strongly positive.
PPD test results are generally analyzed based on several dimensions: induration diameter, presence of blistering or necrosis, grading of positive reaction intensity, interpretation of negative results, and differentiation between false positives and false negatives. The specific analysis is as follows:
1. Induration diameter: The diameter of skin induration (not erythema) is measured. A diameter less than 5 mm is considered negative; 5–9 mm is weakly positive; 10–19 mm is moderately positive; and ≥20 mm, or the presence of blistering or necrosis at the injection site, is strongly positive. The size of the induration reflects the strength of the body's delayed-type hypersensitivity response to tuberculosis bacteria.
2. Presence of blistering or necrosis: Development of a blister larger than 1 mm in diameter at the indurated site, or accompanying skin tissue necrosis or ulceration, indicates a strongly positive reaction. Such manifestations are commonly seen during active tuberculosis infection or in individuals with compromised immunity, requiring further evaluation for tuberculous lesions.
3. Grading of positive reaction intensity: Positive reactions are classified into four grades based on induration size and associated symptoms. A weakly positive result may indicate prior infection or successful BCG vaccination; a moderately positive result requires clinical assessment of infection risk; a strongly positive result highly suggests tuberculosis infection and necessitates immediate further diagnostic workup.
4. Interpretation of negative results: A result is considered negative if the induration diameter is less than 5 mm and there are no local symptoms. However, conditions such as immunosuppression, early stage of infection, or testing errors must be ruled out. False-negative results may occur in newborns, elderly individuals, or those on long-term immunosuppressive therapy.
5. Differentiation of false positives and false negatives: Non-tuberculous mycobacterial infections, cross-reactivity from vaccines, or measurement errors may lead to false-positive results. False-negative results may occur during early infection, in cases of severe tuberculosis, or in individuals with immune deficiencies. Clinical symptoms, imaging findings, and microbiological tests should be integrated for accurate diagnosis.
Results should be read 48 to 72 hours after PPD administration, and scratching the injection site should be avoided. Individuals with positive results should promptly visit designated tuberculosis medical institutions for chest imaging and sputum testing. Even those with negative results should undergo regular follow-up if they develop symptoms such as cough or fever.
