What value generally indicates a positive Tuberculosis (TB) result?

From the Guidelines

A positive tuberculosis (TB) test is generally indicated by an induration of 5 mm or more in diameter for high-risk individuals, 10 mm or more for moderate-risk individuals, and 15 mm or more for low-risk individuals, as measured by the tuberculin skin test (TST) or Mantoux test 1. The interpretation of TB test results varies based on risk factors, including HIV status, recent TB contacts, and chest X-ray findings consistent with prior TB.

• For high-risk individuals, such as those with HIV infection, recent TB contacts, or abnormal chest radiographs, an induration of 5 mm or more is considered positive 1.
• For moderate-risk individuals, such as healthcare workers or those with certain medical conditions, the threshold is 10 mm 1.
• For low-risk individuals, 15 mm or more indicates a positive result 1. It's essential to consider the risk factors and medical history of the individual when interpreting TB test results, as false-negative reactions can occur in certain circumstances, such as immunosuppression or recent viral infections 1. The tuberculin skin test has a high sensitivity and specificity, but its accuracy can be affected by prior BCG vaccination, exposure to nontuberculous mycobacteria, and other factors 1. Interferon-gamma release assays (IGRAs) like QuantiFERON-TB Gold or T-SPOT.TB are alternative tests that can be used to diagnose TB infection, with results reported as positive, negative, or indeterminate based on the amount of interferon-gamma released by T-cells in response to TB antigens 1. A positive TB test indicates infection with Mycobacterium tuberculosis, but it does not necessarily mean the person has active TB disease; it could indicate latent TB infection requiring further evaluation with chest X-rays and sputum tests 1.

From the FDA Drug Label

Isoniazid is recommended as preventive therapy for the following groups, regardless of age. (Note: the criterion for a positive reaction to a skin test (in millimeters of induration) for each group is given in parenthesis): 1 Persons with human immunodeficiency virus (HIV) infection (≥ 5 mm) 2. Close contacts of persons with newly diagnosed infectious tuberculosis (≥ 5 mm) 3 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). 4 Persons with abnormal chest radiographs that show fibrotic lesions likely to represent old healed tuberculosis (≥ 5 mm). 5 Intravenous drug users known to be HIV-seronegative (> 10 mm). 6 Persons with the following medical conditions that have been reported to increase the risk of tuberculosis (≥ 10 mm)

A positive Tuberculosis (TB) result is generally indicated by the following values:

• ≥ 5 mm for persons with human immunodeficiency virus (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, and persons with certain medical conditions that increase the risk of tuberculosis
• ≥ 10 mm increase within a 2-year period for those < 35 years old, and ≥ 15 mm increase for those ≥ 35 years of age, for recent converters 2
• ≥ 15 mm for persons under the age of 35 who have no risk factors and do not belong to high-incidence groups 2

From the Research

Tuberculosis Test Results

• A positive Tuberculosis (TB) result is generally indicated by an induration size of 10 mm or more, as seen in studies 3, 4.
• However, the optimal cutpoint for a positive test result can vary depending on the population being tested and the purpose of the test, ranging from 2 mm to 16 mm 3.
• For individuals who have been vaccinated with bacillus Calmette-Guérin (BCG), a tuberculin skin test induration of 15 mm or more is often used to indicate a positive result 4.
• The QuantiFERON-TB Gold test is another diagnostic tool used to detect latent TB infection, and a result of 0.35 IU/mL or more is generally considered positive 5, 6.

Test Characteristics

• The tuberculin skin test has a sensitivity of 76.5% and a specificity of 77.2% for detecting latent TB infection 6.
• The QuantiFERON-TB Gold In-Tube test has a sensitivity of 68.35% and a specificity of 82.32% for detecting latent TB infection 6.
• The T-SPOT.TB test has a sensitivity of 92% and a specificity of 95.7% for detecting latent TB infection 6.

Diagnostic Algorithms

• A proposed algorithm for latent TB infection screening recommends using the QuantiFERON-TB Gold Plus test as the initial diagnostic tool, followed by the T-SPOT.TB test in cases of indeterminate results 7.
• The T-SPOT.TB test has been shown to be effective in resolving indeterminate QuantiFERON-TB Gold Plus results, with a resolution rate of 84.8% when performed within 30 days of the initial test 7.