TST test and cross-reactivity with the BCG vaccine
Since 1 January 2016, Taiwan Centers for Disease Control (CDC) adjusted the recommended timeframe for BCG vaccination – from 24 hours after birth, or as early as possible, to between 5 – 8 months and no later than 1 years old6. This means that almost all babies are vaccinated BCG. Not all vaccines work equally well, and the BCG has significant limitations in preventing the spread of TB.
TST uses mixture proteins, sometimes referred to as purified protein derivative (PPD), and these protein cross react with BCG and non-tuberculous Mycobacteria (NTM), leading to low specificity in patients with BCG vaccine. Low specificity means higher false positive results especially in children, which leads to overtreatment7.
Limitations with TST that are seen in BCG vaccinated populations are absent when using IGRAs.
IGRAs are an in vitro diagnostic blood test, which detect interferon-gamma released by T cells after exposure to certain TB-specific antigens, as an indirect measure of adaptive cell immunity. The T-SPOT.TB test, which is a type of commercially-available IGRA test, uses TB-specific antigens ESAT-6 and CFP10. These antigens used are important because they are specific for MTB, and not present in the BCG vaccine or non-TB mycobacterium (NTM), limiting false reactions.