Rapid diagnosis of Bacteroides infections by indirect immunofluorescence assay of clinical specimens.

43 specimens from a variety of sites were directly examined by indirect immunofluorescence assay (I.F.A.) with specific antisera against the capsular polysaccharide of Bacteroides fragilis and pooled antisera against a number of serotypes of Bacteroides sp. (all of the former B. fragilis subspecies). The findings were compared with those of routine anaerobic bacteriology and gas liquid chromatography for short chain fatty acids. Examination by I.F.A. was a sensitive (100%) and specific (90.3%) means of identifying B. fragilis. Use of the pooled serum was sensitive (100%) but less specific (64.3%) than the capsular antiserum (90.3%) although it had the advantage of detecting Bacteroides species other than B. fragilis. The capsular serum I.F.A. gave 9.7% false positives and no false negatives. The predictive value of a positive identification of B. fragilis in a clinical specimen using this anticapsular serum I.F.A. test was 80%; with the pooled Bacteroides group-serum it was 60%. The predictive value of a negative test was 100% for both sera, indicating that a negative I.F.A. test is a reliable index of the absence of Bacteroides from the culture I.F.A. of clinical material provides a rapid (less than 2 h) specific and sensitive means for the diagnosis of B. fragilis infections and would be of use in a clinical laboratory.