Tuberculin Skin Test (Mantoux) Interpreter
Tuberculin Skin Test (Mantoux) Interpreter
Apply the correct induration threshold for the patient's risk group, from 5 mm in the highest-risk groups to 15 mm with no risk factors.
Tuberculin Skin Test (Mantoux)
Induration + risk → resultInduration 8 mm, intermediate risk
The threshold depends on the person, not the test
10 mm — recent immigrant, injecting drug use, congregate setting, laboratory staff, predisposing condition
15 mm — no known risk factors
- induration
- measure the raised, palpable swelling, not the surrounding erythema, transversely to the forearm's long axis
- reading time
- 48 to 72 hours after placement
- positive result
- indicates infection, not active disease — exclude active tuberculosis before treating latent infection
- BCG
- causes false positives, especially if given after infancy — IGRAs are preferred in BCG-vaccinated populations
Worked example
Induration 8 mm, intermediate risk
Intermediate risk carries a 10 mm threshold
8 mm is below that threshold → negative
Reconsider if a false-negative cause applies, given the pre-test risk
Thresholds by risk group
| Risk group | Threshold | Examples |
|---|---|---|
| Highest risk | ≥ 5 mm | HIV, recent close contact, organ transplant, immunosuppressive therapy, fibrotic chest X-ray changes |
| Intermediate risk | ≥ 10 mm | Recent immigrant, injecting drug use, congregate living or work setting, laboratory personnel, predisposing medical condition |
| Low risk | ≥ 15 mm | No known risk factors |
Reading the test correctly
The tuberculin skin test does not have a single cut-off; the threshold that defines a positive result depends entirely on the person being tested. Five millimetres is positive in the highest-risk groups — HIV infection, recent close contact with active tuberculosis, organ transplantation, immunosuppressive therapy, or fibrotic changes on chest X-ray consistent with old, untreated tuberculosis — because in these groups the consequence of missing latent infection far outweighs the cost of a false positive. Ten millimetres applies to intermediate-risk groups such as recent immigrants from high-incidence countries, people who inject drugs, residents or workers in congregate settings, and laboratory personnel. Fifteen millimetres applies with no risk factors at all, to minimise false positives.
Technique matters as much as the threshold. Measure the induration — the raised, palpable swelling — transversely to the long axis of the forearm, not the surrounding erythema, and read it at 48 to 72 hours; a reading taken outside that window is unreliable.
A positive result indicates infection, not active disease, and active tuberculosis must be excluded clinically and radiographically before treating latent infection. Prior BCG vaccination, particularly if given after infancy, causes false positives, which is why interferon-gamma release assays are generally preferred in BCG-vaccinated populations. False negatives occur in anergy, in HIV infection with a low CD4 count, in the first 8 weeks after infection before the immune response has matured, after a recent live viral vaccine, and paradoxically in overwhelming tuberculosis itself, where the immune response can be too depleted to react.
Frequently asked questions
What induration counts as positive?
It depends on risk group: 5 mm in the highest-risk groups (HIV, recent contact, transplant, immunosuppression, fibrotic chest X-ray changes), 10 mm in intermediate-risk groups, and 15 mm with no known risk factors.
Should I measure the redness or the swelling?
The swelling — the induration — not the surrounding erythema. Measure it transversely to the long axis of the forearm, read at 48 to 72 hours after placement.
Does a positive test mean active tuberculosis?
No. It indicates infection at some point, not necessarily active disease. Active tuberculosis must be excluded clinically and radiographically before treating for latent infection.
Why does BCG vaccination cause false positives?
BCG contains live attenuated mycobacteria that can cross-react with tuberculin, particularly when given after infancy. This is why interferon-gamma release assays, which do not cross-react with BCG, are generally preferred in vaccinated populations.
Related calculators
References
- Centers for Disease Control and Prevention. Tuberculin Skin Testing — Fact Sheet.
- World Health Organization. Latent Tuberculosis Infection: Updated and Consolidated Guidelines for Programmatic Management.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.