Conversion of Tuberculin Skin Test in Adults

Tuberculosis is a major public health hazard. The tuberculin skin test is one of the diagnostic tools in this regard. Since BCG vaccination is performed during infancy in Iran, a positive tuberculin skin test (PPD) may confuse the physician. For this reason we performed this study on adults. A descriptive study was performed on 433 soldiers between 2006 and 2007. Demographic data like age, level of education, family history of tuberculosis, place of residence, cigarette smoking, and chronic cough for more than 3 weeks, were collected from each patient. All patients had a history of BCG vaccination during infancy and its scar was detected in all of them. Purified protein derivative (PPD) test was performed. A 0.1 ml of 5TU PPD solution was injected intradermally into the volar face of the forearm and after 72h transverse diameter of the induration was measured in millimeters with a transparent ruler. Induration size greater than l0mm was considered a positive reaction. All patients were followed for one year and participated again in the second phase of PPD injection after one year. Data were analyzed by using SPSS software ver.13. Paired t-test and Chi-square test were used for statistical evaluation. All soldiers were male with a mean age of 23.2±1.8 yrs. Twenty-three cases (5.3%) had positive tuberculin skin tests after one year. The highest level of education was high school diploma in 288 soldiers (66.5%). There was no significant correlation between the educational background and positivity of the tuberculin skin test (p=0.219). Twenty-two cases (5%) had a history of cigarette smoking which was significantly related to positive tuberculin skin tests (p=0.001). There was chronic cough in 44 (10.6%) soldiers which did not have any significant correlation with tuberculin skin test results (p=0.6). This study showed that the prevalence of new cases of tuberculosis was more than 5% per year. Therefore, performing tuberculin skin test in BCG vaccinated adults is important.