Purpose and Interpretation of the Mantoux Test for Tuberculosis
The Mantoux test (PPD skin test) is the standard method for identifying latent tuberculosis infection by measuring the immune response to tuberculin protein injected intradermally, with interpretation based on the size of induration and patient risk factors. 1, 2
Administration Technique
- The test is administered using the Mantoux technique, involving intracutaneous injection of 0.1 mL of purified protein derivative (PPD) containing 5 tuberculin units (TU) into the volar or dorsal surface of the forearm 1, 2
- A proper injection should produce a pale elevation of the skin (wheal) measuring 6-10 mm in diameter 1, 2
- The test must be read by trained healthcare personnel between 48 and 72 hours after injection; patient self-reading is not acceptable 1, 2, 3
- Only the induration (hardened area) should be measured in millimeters, not the erythema (redness) 1, 2, 3
Interpretation of Results
The interpretation of PPD test results depends on three key factors:
- The purpose of testing (diagnostic vs. screening)
- The prevalence of TB in the population being tested
- The individual's risk factors for TB infection and progression 1, 2
Risk-Based Interpretation Thresholds
≥5 mm induration is positive in high-risk individuals:
≥10 mm induration is positive in moderate-risk individuals:
- Recent immigrants from high TB prevalence countries
- Injection drug users
- Residents and employees of high-risk congregate settings (prisons, nursing homes)
- Healthcare workers
- Children under 4 years of age
- Persons with clinical conditions that increase TB risk (diabetes, silicosis, end-stage renal disease) 1, 2
≥15 mm induration is positive in persons with no known risk factors for TB 1, 2
Clinical Applications
- The primary purpose is to identify individuals with latent tuberculosis infection who may benefit from preventive therapy 2
- The test is particularly important for screening high-risk populations such as healthcare workers, correctional facility residents, and immunocompromised patients 1, 2
- For healthcare workers, baseline PPD testing should be performed during pre-employment examinations 1, 2
- A positive test requires follow-up with chest radiography and clinical evaluation to exclude active TB before initiating treatment for latent TB infection 1, 2
Special Considerations
Two-Step Testing
- Two-step testing is recommended for initial screening of individuals who will undergo periodic testing (e.g., healthcare workers) 1, 2
- This process identifies the "booster phenomenon" where an initial negative test may stimulate the immune system to respond to a second test 1-3 weeks later 2
- Without two-step testing, this boosted reaction might be misinterpreted as a new infection during subsequent testing 2
BCG Vaccination
- Previous BCG vaccination can cause false-positive PPD results but should not prevent PPD testing in individuals who need screening 2, 3
- The larger the reaction to PPD, the greater the probability of true TB infection rather than BCG effect 3
Common Pitfalls and Limitations
- Improper administration technique can lead to false-negative results 2, 3
- Reading the test outside the recommended 48-72 hour window reduces accuracy 1, 2
- Measuring erythema instead of induration leads to incorrect interpretation 1, 2, 3
- Failure to consider risk factors when interpreting results can lead to inappropriate clinical decisions 1, 2
- The test has limited specificity, especially in BCG-vaccinated individuals 4, 5
- Poor return rates for test reading can be a significant practical limitation in some settings 6
Newer Alternatives
- Interferon-gamma release assays (IGRAs) are newer blood tests that may offer advantages over the Mantoux test, including:
- These tests are increasingly being used, particularly in BCG-vaccinated individuals and settings where return for PPD reading is problematic 5