How do you interpret a Tuberculin (TB) skin test result?

From the Guidelines

To interpret a Tuberculin (TB) skin test result, measure the induration at the injection site 48-72 hours after administration, with a positive result varying based on risk factors: ≥5mm for high-risk individuals, ≥10mm for those with increased probability of recent infection, and ≥15mm for those with no known risk factors. The test is administered by injecting 0.1 ml of 5 tuberculin units (TU) PPD intradermally into the forearm, and the transverse diameter of induration should be recorded in millimeters 1. The sensitivity and specificity of the tuberculin skin test are high, but false-negative reactions can occur in certain populations, such as infants and young children, or those with immunosuppressive conditions 1. Some key points to consider when interpreting TB skin test results include:

• The test works by causing a delayed-type hypersensitivity reaction in individuals previously exposed to tuberculosis bacteria, with the immune system's memory T-cells recognizing the injected tuberculin antigens and triggering local inflammation 1.
• The test should be read within 48-72 hours after administration, as readings taken too early or too late may be inaccurate 1.
• A reaction of 5 mm or greater is considered positive for close contacts of tuberculosis cases, immunosuppressed persons, and individuals with clinical or radiographic evidence of current or prior TB 1.
• A reaction of ≥10 mm is considered positive for other persons at increased risk of LTBI, such as those born in high TB incidence countries or with medical risk factors that increase the probability of progression from LTBI to TB 1.
• A reaction of 15 mm or greater is considered positive for all other persons 1. It's also important to note that the tuberculin skin test has some limitations, such as decreased specificity among persons with prior BCG vaccination or exposure to nontuberculous mycobacteria 1. Overall, the TB skin test is a useful tool for diagnosing latent tuberculosis infection, but its interpretation should be guided by the individual's risk factors and medical history, as outlined in the most recent guidelines 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) and persons with risk factors for HIV infection whose HIV infection status is unknown but who are suspected of having HIV infection. 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)

The interpretation of a Tuberculin (TB) skin test result depends on the individual's risk factors and medical conditions.

• A reaction of ≥ 5 mm is considered positive for persons with HIV infection, close contacts of persons with newly diagnosed infectious tuberculosis, and persons with abnormal chest radiographs showing fibrotic lesions likely to represent old healed tuberculosis.
• A reaction of ≥ 10 mm is considered positive for recent converters, intravenous drug users known to be HIV-seronegative, and persons with certain medical conditions that increase the risk of tuberculosis.
• A reaction of ≥ 10 mm is also considered positive for children under 4 years old.
• For persons under 35 years old with no risk factors, a reaction of ≥ 10 mm is considered positive if they belong to a high-incidence group, and ≥ 15 mm is considered positive if they do not belong to a high-incidence group 2.

From the Research

Interpreting Tuberculin Skin Test Results

The interpretation of Tuberculin Skin Test (TST) results is crucial for the diagnosis of latent tuberculosis infection. The results are measured in terms of the size of induration, which is the firm, raised tissue at the site of the injection.

• The size of induration is measured in millimeters, and the results are classified as follows:
  • Negative: Less than 5 mm of induration
  • Positive: 5 mm or more of induration
• However, the cutoff value for a positive TST result may vary depending on the population being tested and the presence of risk factors for tuberculosis, as seen in studies 3, 4.
• For example, in a study of healthy employees in Iran, a 5-10 mm induration was seen in 276 subjects, while a 10-15 mm induration was seen in 292 subjects 3.
• Another study found that the use of 5 TU PPD at 48 hours by a trained physician can aid in early and more reliable diagnosis of TB, with a tendency to under-read around the cutoff point 5.

Factors Affecting TST Results

Several factors can affect the results of the TST, including:

• Skin thickness: A study found that skin thickness, as measured by the modified Rodnan skin score (mRSS), had a significant negative correlation with the reaction size, making the TST less sensitive for detecting TB infection in patients with systemic sclerosis 6.
• Reader variability: Interobserver variability in reading TST results can lead to differences in interpretation, with a tendency to under-read around the cutoff point 5.
• Time of reading: The size of induration can be similar when read at 48 or 72 hours, but the accuracy of reading can vary depending on the method used, such as the mTST approach 7.

Diagnostic Agreement and Cutoff Values

The diagnostic agreement between TST and other tests, such as QuantiFERON-TB gold in-tube (QFT), can vary, with a study finding a diagnostic agreement of 36.6% between TST ≥10 mm and QFT 4.

• The cutoff value for a positive TST result may need to be adjusted depending on the population being tested and the presence of risk factors for tuberculosis, with a study suggesting that the induration diameter of TST can be reduced from 15 to 9 mm to enhance sensitivity in resource-poor areas 4.