From the Guidelines
A positive PPD test is determined by measuring the size of induration at the injection site after 48-72 hours, with thresholds varying according to risk factors, including 5mm or greater for individuals with HIV, recent TB contacts, or chest X-ray findings consistent with prior TB, 10mm or greater for those with diabetes, kidney failure, immunosuppression, or from high TB prevalence countries, and 15mm or greater for individuals with no known risk factors, as recommended by the most recent guidelines 1.
Key Considerations
- The test is considered positive based on specific measurement thresholds that vary according to risk factors.
- For individuals with HIV, recent TB contacts, or those with chest X-ray findings consistent with prior TB, an induration of 5mm or greater is positive.
- For people with diabetes, kidney failure, immunosuppression, or those from high TB prevalence countries, the threshold is 10mm or greater.
- For individuals with no known risk factors, an induration of 15mm or greater indicates a positive result.
Important Notes
- Only the induration (not redness) should be measured, and the measurement should be taken perpendicular to the long axis of the forearm.
- A positive PPD indicates TB infection but doesn't necessarily mean active disease; further evaluation with chest X-ray and symptom assessment is required to distinguish between latent TB infection and active TB disease.
- False positives can occur in people previously vaccinated with BCG or infected with nontuberculous mycobacteria, as noted in previous studies 1.
Risk Factors and Thresholds
- Individuals with HIV, recent TB contacts, or chest X-ray findings consistent with prior TB: 5mm or greater
- People with diabetes, kidney failure, immunosuppression, or those from high TB prevalence countries: 10mm or greater
- Individuals with no known risk factors: 15mm or greater It is essential to follow the most recent guidelines and consider individual risk factors when interpreting PPD test results, as emphasized in the 2017 guidelines 1.
From the Research
Definition of a Positive PPD Test
A positive Purified Protein Derivative (PPD) test is defined by the appearance of an indurated area of a certain size, measured in millimeters, 48 to 72 hours after administration of the PPD. The size of the induration that constitutes a positive reaction varies depending on the individual's risk factors and medical history.
Induration Sizes for Positive Reactions
- 5 mm or more is considered a positive reaction in persons who have recently had close contact with an individual with active tuberculosis, in persons with radiographic findings consistent with a past history of tuberculosis, or in persons with known or suspected human immunodeficiency virus infection 2.
- 10 mm or more is considered a positive reaction in persons at increased risk of tuberculosis 2, 3.
- 15 mm or more is considered a positive result in all other persons 2, 4.
Factors Influencing Test Results
The interpretation of PPD test results can be influenced by various factors, including:
- History of contact with a smear-positive case 4.
- Clinical findings of tuberculosis 4.
- Immunosuppressive conditions, which can diminish the size of induration in an infected child 4.
- Type of purified protein derivative used for the test, as different PPDs can produce different induration sizes and IGRA confirmation rates 5.
- BCG vaccination status, as BCG-vaccinated individuals may have different induration sizes and IGRA confirmation rates compared to non-BCG-vaccinated individuals 5.