IgG antibody to purified protein derivative by enzyme-linked immunosorbent assay in the diagnosis of pulmonary tuberculosis.

We studied the diagnostic utility of an enzyme-linked immunosorbent assay (ELISA) in hospitalized patients with suspected pulmonary tuberculosis (TB). A positive culture for M. tuberculosis identified active disease, and 3 negative cultures and smears defined the negative group. IgG antibody activity was determined by adding a 1:1,000 dilution of serum to plates coated with PPD antigen. Alkaline phosphatase labeled anti-IgG was added, color developed, and an optical density index (ODI) was determined. Twenty-one patients with M. tuberculosis TB had a mean ODI of 0.27, which was higher than 99 patients without TB, ODI 0.10 (p less than 0.001). An ODI of 0.15 or greater was established as a positive ELISA test. For patients with M. tuberculosis TB, the ELISA had a sensitivity of 67 and a specificity of 79%. The first smear had a sensitivity of 57 and a specificity of 99%. With the first smear and the ELISA test results, a combined sensitivity of 86% was achieved. When both ELISA and the first smear were negative (101 cases), active TB caused by M. tuberculosis was found in only three patients (3.0%).