Tuberculin reactivity in adult BCG-vaccinated subjects: a cross-sectional study.

INTRODUCTION

Interpretation of the tuberculin skin test (TST) may be complicated by prior bacille Calmette-Guérin (BCG) vaccination. The skin reaction to the vaccination interferes with the management of individuals who may be infected with Mycobacterium tuberculosis.

OBJECTIVE

To discriminate between TST reactions due to infection and those due to vaccination in subjects with unknown BCG status.

METHODS

Among 60200 subjects tested with 5TU PPD for screening purposes, 4987 contacts of infectious TB cases (Group A), 4962 BCG-vaccinated subjects (Group B) and 5000 subjects from the general population (Group C) were sampled. The frequencies of TST cut-off diameters were calculated for the three groups using a logistic regression model. The frequency of positive subjects in each group and the sensitivity, specificity and predictive values were also computed by means of these cut-offs.

RESULTS

The risk of being a contact versus BCG-vaccinated increases 2.43-fold with every mm of TST diameter. The 11 mm cut-off point seems to be the best discriminating value.

CONCLUSIONS

Using the traditional 10 mm cut-off, we can consider all vaccinated subjects with a positive TST to be infected. The TST remains a valuable tool for the evaluation of household contacts and suspected cases of tuberculosis in BCG-vaccinated subjects and in populations with high vaccination coverage.