What is the cutoff size for a positive Mantoux (Tuberculin) TB (Tuberculosis) skin test?

From the Guidelines

The cutoff size for a positive Mantoux tuberculosis skin test varies based on the person's risk factors, with a cutoff of 5 mm or more considered positive for individuals with HIV, recent TB contacts, or those who are immunosuppressed, 10 mm for those with moderate risk factors, and 15 mm for those with no known risk factors. When interpreting the results of a tuberculin skin test, it is essential to consider the individual's risk factors for tuberculosis (TB) infection. The test involves injecting purified protein derivative (PPD) under the skin, which causes a delayed-type hypersensitivity reaction in people previously exposed to TB bacteria 1. The reaction must be read 48-72 hours after placement by measuring the diameter of induration perpendicular to the long axis of the forearm. Some key points to consider when interpreting the results of a tuberculin skin test include:

• For individuals with HIV, recent TB contacts, organ transplant recipients, or those who are immunosuppressed, a smaller induration of 5 mm or more is considered positive 1.
• For people with moderate risk factors such as diabetes, kidney disease, or those from high TB prevalence countries, the threshold is 10 mm 1.
• For those with no known risk factors, 15 mm or larger is considered positive 1. It's crucial to note that a positive test indicates TB infection but doesn't necessarily mean the person has active TB disease; further evaluation with chest X-ray and other tests is needed to determine if treatment is required. In terms of specific guidelines, the American Thoracic Society and the Centers for Disease Control and Prevention recommend targeted tuberculin testing and treatment of latent tuberculosis infection, with a focus on high-risk populations such as those with HIV, recent TB contacts, and those who are immunosuppressed 1. Additionally, health-care facilities and other high-risk institutions should conduct a risk assessment to determine the frequency of testing for M. tuberculosis infection among employees, as a component of the proper level of TB infection control measures 1.

From the FDA Drug Label

Persons with human immunodeficiency virus (HIV) infection (≥ 5 mm) and persons with risk factors for HIV infection whose HIV infection status is unknown but who are suspected of having HIV infection. Close contacts of persons with newly diagnosed infectious tuberculosis (≥ 5 mm) Recent converters, as indicated by a tuberculin skin test (≥ 10 mm increase within a 2-year period for those < 35 years old; ≥ 15 mm increase for those ≥ 35 years of age). Persons with abnormal chest radiographs that show fibrotic lesions likely to represent old healed tuberculosis (≥ 5 mm). Intravenous drug users known to be HIV-seronegative (> 10 mm). Persons with the following medical conditions that have been reported to increase the risk of tuberculosis (≥ 10 mm) Children who are less than 4 years old are candidates for isoniazid preventive therapy if they have > 10 mm induration from a PPD Mantoux tuberculin skin test persons under the age of 35 who ... have a tuberculin skin test reaction of 10 mm or more persons under the age of 35 who ... have a tuberculin skin test reaction of 15 mm or more

The cutoff size of a positive Mantoux TB skin test is:

• ≥ 5 mm for persons with HIV infection, close contacts of persons with newly diagnosed infectious tuberculosis, and persons with abnormal chest radiographs that show fibrotic lesions likely to represent old healed tuberculosis.
• ≥ 10 mm for recent converters, intravenous drug users known to be HIV-seronegative, persons with medical conditions that increase the risk of tuberculosis, children under 4 years old, and persons under 35 years old.
• ≥ 15 mm for persons under 35 years old with no risk factors and no high-incidence groups, and for recent converters over 35 years old 2

From the Research

Cutoff Size of Positive Mantoux TB Skin Test

• The cutoff size of a positive Mantoux TB skin test can vary depending on the population being tested and the purpose of the test 3.
• In the general US population, a cutoff of 10 mm is often used, but this can be adjusted based on the prevalence of TB in the population and the individual's risk factors 3.
• For individuals with HIV infection, a lower cutoff of 5 mm may be used to compensate for the decreased sensitivity of the test in this population 4.
• However, reducing the cutoff value from 10 mm to 5 mm may not significantly increase the sensitivity of the test in HIV-infected individuals, as the loss of sensitivity is often due to anergy rather than a gradual decrease in skin test responsiveness 4.
• In some studies, a cutoff of 10-12 mm has been suggested as a way to improve the specificity of the test while maintaining adequate sensitivity 5.
• The optimal cutoff point may also depend on the individual's age, with older individuals potentially requiring a higher cutoff to avoid false positives 6.
• Ultimately, the interpretation of the Mantoux skin test results should take into account the individual's risk factors, medical history, and the prevalence of TB in the population being tested 3.

Key Findings

• A study in Tanzania found that the sensitivity of the TST among HIV-uninfected patients was 91.1% at a cutoff value of 10 mm and 95.2% at a cutoff value of 5 mm 4.
• The same study found that the sensitivity of the TST among HIV-infected patients was 64.3% at a cutoff value of 10 mm and 71.2% at a cutoff value of 5 mm 4.
• A study in Eastern China found that the diagnostic value of the TST reached its highest point when the induration diameter was 10.25 mm, with a sensitivity and specificity of 0.731 and 0.727, respectively 5.
• A study in Spain found that the induration size was significantly smaller in HIV-positive individuals than in HIV-negative individuals, and that the booster effect was detected in 8.9% of HIV-positive individuals 6.