Nonspecificity of assaying for IgG antibody to pneumolysin in circulating immune complexes as a means to diagnose pneumococcal pneumonia

Detection of immunoglobulin G (IgG) antibody to pneumolysin (PLY) in precip itated circulating immune complexes (CICs) has been used to diagnose pneumo coccal pneumonia. With care to include appropriate controls, we precipitate d and dissociated CICs and then assayed for IgG antibody to PLY. We detecte d IgG antibody to PLY in CICs that were precipitated from serum samples tha t were obtained at the time of admission to the hospital from 5 (23%) of 22 healthy adults, 7 (44%) of 16 subjects with stable chronic obstructive pul monary disease, 10 (63%) of 16 subjects colonized with Streptococcus pneumo niae, and 9 (60%) of 15 patients with nonbacteremic pneumococcal pneumonia. Of the 16 patients with bacteremic pneumococcal pneumonia, 4 (25%) had IgG antibody to PLY at the time of admission, and 8 (50%) had IgG antibody to PLY in convalescence. Levels of IgG antibody in CICs closely correlated wit h serum levels of IgG antibody to PLY, implicating precipitation of free se rum antibody in tests with false-positive results. Detection of IgG antibod y to PLY in precipitated CICs is not a reliable method for diagnosing pneum ococcal pneumonia.