MITARAI, S., OISHI, K., FUKASAWA, M., YAMASHITA, H., NAGATAKE, T. and MATSUMOTO, K. Clinical Evaluation of Polymerase Chain Reaction DNA Amplification Method for the Diagnosis of Pulmonary Tuberculosis in Patients with Negative Acid-fast Bacilli Smear. Tohoku J. Exp. Med., 1995, 177 (1), 13-23-We evaluated the sensitivity and specificity of polymerase chain reaction (PCR) for the detection of Mycobacterium tuberculosis among 109 patients who were suspected to have active pulmonary tuberculosis (PTB) and showed negative acid-fast bacilli (AFB) smears in a total of 393 samples of sputum (169), gastric aspirate (134), and urine (90) which fulfilled different criteria for the positivity of PCR. The patients were subsequently divided into one group of active PTB composed of 15 patients with definite PTB and 43 patients with highly susp ectedPTB, and another group of 51 non-active PTB patients. The PCR assay using samples of sputum and gastric aspirate proved to be specific for active PTB. The PCR method for diagnosis of active PTB using sputum samples was sensitive (97.8%) but lacked specificity (27.0%) when regarded as PCR positive when at least one positive reaction was obtained among all samples examined. However, the PCR of gastric aspirate demonstrated a sensitivity of 63.4% and a specificity of 76.7%. Our data supports that the PCR method for detecting active PTB in AFB smear negative patients using gastric aspirate shows markedly improved sensitivity over the conventional method (25.9%), although it still lacks specificity. PCR assay for M. tuberculosis using multiple samples of gastric aspirate in conjunction with careful clinical observations for the presence of active infection is essential for the diagnosis of active PTB among patients with negative AFB smear.