A PARTICLE COUNTING IMMUNOASSAY FOR THE DIRECT-DETECTION OF CLOSTRIDIUM-DIFFICILE SEROGROUP SPECIFIC ANTIGEN IN FECAL SPECIMENS

The potential of a particle counting immunoassay (FACIA) for the direct detection of Clostridium difficile serogroup G specific antigen in faecal specimens was evaluated. F(ab'), fragments from a rabbit anti-serogroup G antiserum were covalently coupled to carboxylated latex beads. This reagent was mixed with acid extracts of faecal specimens and the reaction was assayed with an optical counter which discriminated unagglutinated from agglutinated latex particles. Culture for C. difficile, faecal cytotoxin detection, FACIA and serogrouping of C. difficile isolates were performed on 249 stools. Of the 71 culture-negative specimens, none gave a positive result in the cytotoxin assay or in FACIA. Faecal cytotoxin was detected in 100 of the 178 culture-positive specimens. FACIA was positive for 63 of the 71 faecal specimens that yielded serogroup G C. difficile on culture. FACIA gave negative results for all other culture-positive stools tested with one exception, from which a serogroup A7 C. difficile strain was isolated. FACIA detection of serogroup G antigen in faecal specimens showed a sensitivity of 88.7 %, a specificity of 99.7 %, a predictive value of a positive culture with a serogroup G strain of 98.4 %, and a predictive value for specimens that were culture-negative for a serogroup G strain of 95.6 %. The results indicate that FACIA with specific antiserum is a rapid and reliable method for detecting serogroup specific antigens of C. difficile in faecal specimens. Clinical applications of the method are discussed.