From the Guidelines
True, the CDC recommends ignoring a history of BCG vaccination when interpreting tuberculin skin test (TST) results. Healthcare providers should proceed with TST administration and interpret the results based on the patient's risk factors for TB infection and progression to TB disease, regardless of prior BCG vaccination status, as stated in the guidelines from the American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America 1. If the TST is positive according to the appropriate cut-off points (≥5mm, ≥10mm, or ≥15mm depending on risk factors), treatment for latent tuberculosis infection (LTBI) should be offered if applicable. This recommendation exists because the protective effect of BCG vaccination wanes over time, usually within 5-10 years, and a positive TST in a BCG-vaccinated person may still indicate true TB infection.
Some key points to consider when interpreting TST results include:
- The degree of sensitivity to tuberculin that is acquired after BCG vaccination is highly variable, and no reliable method exists for distinguishing tuberculin reactions caused by previous BCG vaccination from those caused by natural mycobacterial infections 1.
- Positive tuberculin reactions in BCG-vaccinated persons from high prevalence areas usually indicate infection with M. tuberculosis, and such persons should be evaluated for preventive therapy 1.
- Most BCG-related TST reactions are smaller than 10mm and typically diminish over time, so larger reactions are more likely to represent actual TB infection rather than BCG cross-reactivity.
- For patients with a history of BCG vaccination, interferon-gamma release assays (IGRAs) like QuantiFERON-TB Gold or T-SPOT.TB may be preferred alternatives as they are not affected by prior BCG vaccination.
The guidelines for the treatment of tuberculosis also emphasize the importance of considering risk factors for TB infection and progression to TB disease when interpreting TST results, and recommend tuberculin skin testing for all patients with a history of one or more of the following: HIV infection, injection drug use, homelessness, incarceration, or contact with a person with pulmonary TB 1.
From the Research
CDC Recommendations on BCG Administration and Tuberculin Skin Test
- The CDC recommends ignoring the history of BCG administration and proceeding with the tuberculin skin test (TST) in certain cases, as there is no evidence that BCG vaccination interferes with TST results in children over 3 years old 2.
- A study found that the risk of a false-positive TST result due to BCG vaccination disappears 3 years after vaccine administration, supporting the CDC's recommendation 2.
- However, under 3 years of age, BCG vaccination may interfere with TST results and cause false-positive results, in which case the use of interferon-gamma release assays (IGRAs) is recommended 2.
- If IGRAs are not available or results are indeterminate, ignoring the history of BCG vaccination is recommended 2.
Interpretation of TST Results
- The interpretation of TST results can be complex and influenced by various factors such as age, immunological status, and coexisting illnesses 3, 4.
- A study found that the Mantoux test has low accuracy in diagnosing doubtful cases of cutaneous tuberculosis, with a sensitivity of 58.97% and specificity of 62.50% 5.
- Another study compared the Mantoux and Tine tuberculin skin tests in BCG-vaccinated children and found substantial agreement between the two tests, but also noted that the Tine test demonstrated a higher positive test rate than the Mantoux test 6.
Treatment Recommendations
- The CDC recommends offering treatment if the TST result is positive, regardless of the history of BCG vaccination 2.
- Treatment decisions should be based on the interpretation of TST results, taking into account various factors that may influence the outcome, and considering the use of IGRAs or other diagnostic tests when necessary 2, 3, 4.