تعيين ارزش اخباري و نقطه برش تست پوستي سل در بيماران مشكوك به سل ريوي شهرستان اراك 1385-1388

Predictive Value and Cut Off Point of PPD in Probable Patients with Pulmonary Tuberculosis

Background and objective: 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-tuberculosis mycobacterium. 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). Materials and 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>5mm were 80%, 61%, 28% and 94% respectively. Sensitivity, specificity, PPV and NPV for PPD>10mm were 53%, 82%, 35% and 90% respectively. The calculated cut off point for PPD was > 4 mm. Conclusion: Our results emphasized that a TST reaction smaller than 5 mm was seen in patients with bacteriological confirmed pulmonary tuberculosis. Predictably, as the cut-off is moved to larger reactions, sensitivity decreases and specificity increases. More estimates are needed to identify the accurate cut off point of PPD in tuberculosis patients.