There is no reliable method of distinguishing tuberculin reactions caused by vaccination with BCG from those caused by natural mycobacterial infections. Therefore, it is usually prudent to consider large reactions to 5 US Units (TU) of Purified Protein Derivative (PPD) tuberculin in BCG-vaccinated persons as indicating infection with TB, especially among persons from areas with a high prevalence of tuberculosis.
There are several reasons for not assuming that a large reaction to tuberculin is due to BCG vaccination:
- Tuberculin test conversion rates after vaccination may be much less than 100%;
- The mean reaction size among vaccines is often less than 10 mm; and
- Tuberculin sensitivity tends to wane after vaccination.
Because many BCG-vaccinated persons tend to come from areas of the world where transmission frequently occurs, it is important that previously vaccinated persons with significant reactions to a tuberculin skin test be evaluated for presence of disease and managed accordingly.
Source: The Role of BCG Vaccination in the Prevention and Control of Tuberculosis in the United States. CDC, MMWR Vol. 45, No RR-4, April 26, 1996.