Evaluation of some methods for the laboratory examination of sputum

Abstract

Methods for the quantitation of leucocytes, squamous epithelial cells, and potential pathogens in sputa are described. Microscopic examination showed that 58% of sputa tested (554/957) were purulent or moderately purulent and 48% were moderately or heavily contaminated by squamous epithelial cells. The presence of squamous epithelial cells indicated oropharyngeal contamination. A simple dilution technique was chosen to compare the isolation of potential pathogens from direct cultures and from dilutions of sputa (10(-7) per ml original sputum). The dilution technique permitted easier reading of sputum cultures and avoided the possible over-reporting of enterobacteria and Pseudomonas aeruginosa, which were frequently found on direct examination. Enterobacteria and Pseudomonas aeruginosa were more likely to be isolated from sputa moderately or heavily contaminated with squamous epithelial cells. The use of counter-current immunoelectrophoresis (CIE) and co-agglutination tests to detect pneumococcal antigen in sputa is described. The presence of antigen in sputum was a more reliable index of lower respiratory tract infection than a positive culture. Co-agglutination tests were simpler to perform and used smaller amounts of expensive antiserum than CIE.