The tuberculin skin test in confirmed pulmonary tuberculosis in the state of Qatar: where we stand?

OBJECTIVE This is the first paper to evaluate the potency of the tuberculin skin test (TST) results in confirmed pulmonary tuberculosis in the developing country. METHOD Data was collected retrospectively from the tuberculosis (TB) treatment unit during the period from 1998 to 2004. All charts diagnosed as active tuberculosis based on positive sputum smear or culture with documented TST were reviewed. The standard TST was done by injecting o.1 ml of 5 international units subcutaneous RT 23 purified protein derivative (PPD) on volar surface of the right arm. RESULTS There were 306 patients with confirmed active pulmonary tuberculosis, of which 58% were smear positive and 42% were smear negative but culture positive. Expatriates accounted for 81% (247) and male for 74% (225) of the patients. The mean TST was 18.5 mm with standard deviation of 7.54 mm. TST was less than 5 mm in 8.2% (25), 5-10 mm in 1.6% (5), 10-15 mm in 11.1% (34) and more than 15 mm in 79.1% (242). False negative (reaction less than 10 mm) was significantly higher in Qatar nationals (17% vs 8% in expatriates) and those with associated diseases (16.3% vs 7% without associated diseases). CONCLUSION Although 9.8% of confirmed pulmonary tuberculosis had false negative TST, it remains a potent aide for epidemiological and diagnostic purposes and periodic assessment of this is highly recommended. In our community with BCG vaccination a reaction more than 10 mm should be considered positive.