Predictive value and cut off point of PPD in probable patients with pulmonary tuberculosis

Abstract

Background: Tuberculin skin testing (TST) is used for identification of tuberculosis infection. Interpretation of test is related to many factors such as prevalence of disease in community, use of vaccine, age of patients and exposure to non-tuberculous mycobacteria. Cut off point of TST is reported between 2 to 16 mm in different communities. In this study we aimed to determine the predictive value and cut off point of TST in probable patients with pulmonary tuberculosis (TB).Methods: A total of 714 probable patients with pulmonary tuberculosis with mean age 57 ± 20.11 years were included in this study. 52.9% of them were male. All subjects were tested using 5 tuberculin units of purified protein derivative (PPD). Gold standard for diagnosis of TB was considered positive smear or culture. Specificity, sensitivity, positive predictive value (PPV) and negative predictive value (NPV) of TST were determined. Cut off point of PPD was determined using Roc curve.Results: 113 patients from 714 suspicious subjects (15.82%) had pulmonary tuberculosis. 80% of them had positive sputum smear. Sensitivity, specificity, PPV and NPV for PPD > = 5 mm were 80%, 61%, 28% and 94% respectively. Sensitivity, specificity, PPV and NPV for PPD > = 10 mm were 53%, 82%, 35% and 90% respectively. The calculated cut off point for PPD was > = 4 mm.