Accuracy of the tuberculin skin test for diagnosis of latent tuberculosis in population with high coverage of Bacillus CalmetteÓ?Guérin vaccination

The TST was administered to 113 cases with PTB and 194 household contacts. The gold standard was the bacteriological confirmation in patients. The household contacts were classified as true positives according to the selected cutoff point. Those with purified protein derivative reactivity less than the cutoff point but different from zero were classified as false positives. The area under the curve was measured and the best cutoff point was determined. Sensitivity, specificity, positive and negative predictive values (PV), and positive and negative likelihood ratios (LR) were measured with 2 2 contingency tables. Results: The best cutoff point was at 10 mm of induration, with an area under the curve of 0.93, sensitivity 85%, specificity 88%, positive PV 94%, negative PV 71%, positive LR 6.9, and negative LR 0.17. There was no association between reactivity to TST and BCG vaccination, OR (95% confidence interval CI) 1.14 (0.65-2.10), p = 0.644. Conclusions: The TST showed a very good performance, as it lead to an important change from pre-test to post-test probability.