The Mantoux Test: Purpose and Interpretation for Tuberculosis Screening
The Mantoux test (PPD skin test) is the standard method for detecting latent tuberculosis infection, serving as a critical screening tool to identify individuals who may benefit from preventive therapy before developing active tuberculosis disease. 1
Administration Procedure
- The Mantoux test is administered by injecting 0.1 ml of PPD (5 tuberculin units) intradermally into the volar or dorsal surface of the forearm, creating a 6-10 mm wheal 2, 1
- The test must be read by trained healthcare personnel between 48-72 hours after injection 2, 1
- Only the induration (hardened area) should be measured in millimeters, not the erythema (redness) 2, 3
- Patient self-reading of results is unreliable and not recommended 2, 1
- If the injection is administered incorrectly (subcutaneously or leaking from site), the test should be repeated immediately at another site 3
Interpretation of Results
The interpretation of PPD reactions depends on the risk profile of the individual being tested:
≥5 mm induration is positive for high-risk individuals:
≥10 mm induration is positive for moderate-risk individuals:
≥15 mm induration is positive for persons with no known risk factors for TB 2, 1
Clinical Applications
- The primary purpose is to identify latent TB infection in individuals who would benefit from preventive therapy 1
- Essential for screening high-risk populations including healthcare workers, correctional facility residents, and immunocompromised patients 2, 1
- Used in contact investigations after exposure to infectious TB cases 2
- Helps guide decisions about preventive therapy to reduce progression to active TB disease 2
Follow-up Actions Based on Results
- Negative test: No further action needed if the individual is not at high risk 1
- Positive test: Requires chest radiography and clinical evaluation to exclude active TB before considering treatment for latent TB infection 2, 1
- Contacts with initial negative tests: Should be retested 8-10 weeks after exposure ends, as conversion may be delayed 2
Special Considerations
Two-Step Testing
- Recommended for baseline testing of healthcare workers and others who will undergo serial testing 1
- Addresses the "booster phenomenon" where a first negative test may stimulate immunologic memory, causing a positive reaction on subsequent testing that could be misinterpreted as new infection 1
- Second test should be administered 1-3 weeks after the first if initial test is negative 1
BCG Vaccination
- History of BCG vaccination is not a contraindication to PPD testing 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 1, 3
- Reading outside the 48-72 hour window reduces accuracy 2, 1
- Poor return rates for test reading (up to 60% in some studies) compromise effectiveness 4
- Limited specificity due to cross-reactivity with BCG vaccination and non-tuberculous mycobacteria 5
- Newer interferon-gamma release assays (IGRAs) may be more specific, especially in BCG-vaccinated individuals 6, 5
- Failure to consider risk factors when interpreting results can lead to inappropriate clinical decisions 1
Emerging Alternatives
- Interferon-gamma release assays (IGRAs) are newer blood tests that measure T-cell response to TB-specific antigens 6, 5
- IGRAs have higher specificity than the Mantoux test, particularly in BCG-vaccinated individuals 5
- IGRAs require only a single visit, which may improve compliance compared to the Mantoux test 1