Clinical and Laboratory Diagnosis of the Patients with Sputum Smear-negative Pulmonary Tuberculosis

The objective of this study is to describe the proportion of patients with chronic cough and negative smear microscopy appropriately diagnosed as tuberculosis (TB) and to identify clinical features that could be used in developing a diagnostic scoring system for smear-negative patients. Records of patients with chronic cough and ≥ 3 negative sputum smears for acid fast bacilli who attended a reference University hospital in south-eastern Iran and screened by culture were retrospectively reviewed. We compared confirmed smear-negative pulmonary TB (PTB; culture-positive) and unconfirmed smear-negative patients (culture-negative) to describe the appropriateness of treatment and their characteristics. Features independently predictive of smear-negative PTB (SNPTB) were entered into a logistic regression to create a diagnostic rule. This study enrolled 350 patients, of which 52 (14.8%) were culture-positive and 298 (85.2%) culture-negative. Of these, 38 out of 52 (sensitivity 73%) confirmed SNPTB were diagnosed as TB and 283 out of 298 (specificity 95%) unconfirmed sputum-negative patients were diagnosed as non-PTB. Variables associated with confirmed SNPTB were the presence of night sweats, family history of TB, typical chest radiography, erythrocyte sedimentation rate > 45 mm and white blood cell count < 11000/mL. The score constructed with these variables had a sensitivity of 94% and specificity of 74% with an area under the curve of 0.90. The clinical differences between SNPTB and control patients could be used to develop a clinical scoring system to identify patients with SNPTB.