The role of antigen detection in pneumococcal carriers: a comparison between cultures and capsular antigen detection in upper respiratory tract secretions

Abstract

During the winter season upper respiratory tract secretions from 166 patients with stable chronic obstructive pulmonary disease (COPD) or asthma were simultaneously cultured for Streptococcus pneumoniae and tested for pneumococcal capsular antigen. Latex agglutination was employed to investigate the effect of pneumococcal carriership on pneumococcal capsular antigen detection in upper respiratory tract secretions. All specimens originating from the oropharynx, nasopharynx and saliva were both cultured and investigated in parallel for the presence of antigen. The recovery of pneumococci from the different areas was unequally distributed (oropharynx 29%, nasopharynx 8%, and saliva 16%), with the highest isolation rate from the oropharynx alone. Only 4 (3%) of the oropharyngeal swabs, 1 (1%) of the nasopharyngeal swabs and 14 (9%) of the saliva specimens yielded both pneumococcal antigen and a positive culture for S. pneumoniae. A further 9 (6%) of the oropharyngeal swabs, 5 (3%) of the nasopharyngeal swabs, and 50 (33%) of the saliva specimens were antigen positive only, with no pneumococci isolated on culture. It is speculated that these reactions were due to cross-reacting microorganisms (especially alpha-haemolytic streptococci) present in saliva and contaminating the oropharynx and the nasopharynx. Quantitative cultures of 9 oropharyngeal swabs yielded S. pneumoniae in concentrations too low to be detectable by latex agglutination. The study indicates that there is a poor relation between pneumococcal colonization and antigen detection in the oropharynx and nasopharynx. Antigen present in these secretions is probably not an important disrupting factor by contamination when detecting pneumococcal antigen in washed sputum. The false positive antigen results in saliva are probably due to cross-reactions with alpha-haemolytic streptococci.