The value of counting BCG scars for interpretation of tuberculin skin tests in a tuberculosis hyperenclemic shantytown, Peru

SETTING: The tuberculin skin test (TST) is widely used as a diagnostic or screening test for Mycobacterium tuberculosis infection and disease. A peri-urban shanty-town in the desert hills of south Lima, Peru, highly endemic for tuberculosis, and where bacille Calmette-Guerin (BCG) vaccine had been given in multiple doses until 1995. OBJECTIVE: To analyze the effect of multiple BCG vaccines on TST in a community-based setting. DESIGN: Point-prevalence survey of TST reactions of 572 people aged 6-26 years from 255 households. TST reactions were compared to the observed number of BCG scars and other potential risk factors (age, living with a TST-positive person, and contact with active tuberculosis). RESULT: People with two or more scars had significantly larger reactions, even after adjusting for potential risk factors. The adjusted population attributable fraction of being TST-positive and having two or more BCG scars was 26%. CONCLUSION: There is no demonstrated benefit of repeat BCG vaccination. We therefore recommend that physicians take into consideration the number of BCG scars when interpreting the TST and that programs give no more than one BCG vaccination.