Determination of antibodies to pneumococcal C polysaccharide in patients with community-acquired pneumonia.

The pneumococcal C polysaccharide (PnC) is species specific and believed to be a cell wall component of all capsular types. Antibodies against PnC in human sera have been demonstrated previously, but the question of whether a rise in these antibodies occurs during pneumococcal infections has not been investigated. We used an indirect enzyme-linked immunosorbent assay (ELISA) for the estimation of PnC antibodies in 124 hospital-treated patients with pneumonia. In 3 of 6 patients with pneumococcal bacteremia and in 17 of 44 patients with S. pneumoniae isolated in the blood, sputum, or nasopharynx, a significant rise in antibody levels was recorded, accounting for a sensitivity of 38.6%. Of 35 patients with pneumonia of other known or suspected etiology, 1 gave a positive result, corresponding to a specificity of 97.1%. In addition, 3 of 8 patients with PnC antigen in the sputum as the only etiological finding and 5 of 37 patients with unknown etiology gave positive results. The PnC antibodies did not seem to have any protective capacity against pneumonia caused by pneumococci. The ELISA, in which only one antigen preparation was used, was more simple than other tests in which traditional capsular antigen preparations are used. It might therefore be used as a supplemental method in the diagnosis of pneumococcal pneumonia. The problems involved in expressing serum titers obtained with the ELISA are discussed.