Simplified coagglutination test for serological grouping of beta-hemolytic streptococci

Streptococci from primary isolation plates grouped by reverse passive haemagglutination

Reverse passive haemagglutination, a novel microtitre based assay, was compared with the Streptex (Wellcome UK) latex slide agglutination kit for streptococcal grouping in a diagnostic microbiology laboratory. Three hundred and fifty two extracts from 349 consecutive primary isolation plates were assayed by both methods. Reverse passive haemagglutination gave identical grouping results for 98.0% of the 345 streptococci identified by Streptex, and the kappa coefficient of agreement between the methods for all 352 extracts tested was 0.973. Cross reactions with Listeria spp seen with Streptex were not found by reverse passive haemagglutination. In the reverse passive haemagglutination method 11 streptococci could be grouped on each 96-well plate and most reactions were stable for at least 30 minutes. Reverse passive haemagglutination is more rapid to perform than latex slide agglutination when many organisms are to be grouped, and the patterns of haemagglutination are easily recognised. If the method was taken into routine use in a diagnostic laboratory, the persistence of reverse passive haemagglutination reactions would enable grouping results to be checked for quality control purposes.

Evaluation of rapid, commercial latex techniques for serogrouping beta-hemolytic streptococci

The clinical need to rapidly and correctly differentiate beta-hemolytic streptococci into Lancefield groups has prompted the development of commercially available rapid agglutination techniques. A modified Streptex (Wellcome Diagnostics, Research Triangle Park, N.C.) technique and the PathoDx latex Strep Grouping Kit (Diagnostic Products Corporation, Los Angeles, Calif.) technique were applied to 220 strains of beta-hemolytic streptococci that were serologically grouped by standard techniques. Agreements between standard and modified Streptex and PathoDx techniques were 99.1 and 100%, respectively. Modified Streptex produced a false-negative for one group G isolate and a weak-positive reaction for a group G reagent with a nongroupable isolate. Sixty-five strains representative of bacteria that may be found in the posterior pharynx in concentrations high enough to cause potential reactions with antigen detection reagents were tested with Streptex and PathoDx reagents. No cross-reactions were observed with any reagent tested when challenged with these 65 strains. When combined with colonial morphology and hemolytic reaction, both modified Streptex and PathoDx were rapid, specific tests for identifying streptococci, with PathoDx being slightly faster.

Detection of bacterial antigens in body fluids by the Phadebact system

150 infected patients from 2 study centers had body fluids examined for presence of bacterial antigens (group B streptococcus, Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae type b) using the Pharmacia Phadebact system. The rates of detection of the pneumococcal antigens in the urine or serum of the patients with pneumococcal pneumonia, pneumococcal bacteremia and pneumococcal meningitis were 0.38, 0.47 and 0.5, respectively. The overall detection rate for H. influenzae infections was 0.92 and for group B streptococcal infections 0.87. Serum and urine from 100 non-infected patients were also tested for the presence of group B streptococcus, S. pneumoniae, N. meningitidis and H. influenzae type b antigens with only 1 false positive (H. influenzae type b).

Comparison of latex agglutination and immunofluorescence for direct Lancefield grouping of streptococci from blood cultures

Simulated positive blood cultures with 84 known stock strains of streptococci were used to comparatively evaluate the direct identification of these organisms by fluorescein-tagged antibody staining (immunofluorescence [IF]) and latex agglutination (LA). IF was not evaluated for Lancefield group D strains (a total of 81 strains tested) and had 89% sensitivity and 91% specificity. IF was least sensitive for the identification of Lancefield group F, in which three of seven strains showed no fluorescence with the group F reagent. Since LA was more convenient and revealed comparable sensitivities and specificities on 84 simulated cultures, we tested this procedure using an additional 29 fresh positive clinical blood cultures, for a total of 113 cultures tested by this technique. Of 11 Streptococcus pneumoniae strains, 9 reacted with the LA group C reagent, a problem not observed with IF. However, all these strains were identified by a rapid modified bile solubility test. Of the 12 Streptococcus faecalis strains, 4 were falsely negative with the group D reagent, but all were correctly identified by a rapid litmus milk reduction test. Of 12 group A strains, 1 was not detected. Of all 113 strains tested by LA, eliminating S. faecalis and S. pneumoniae, the sensitivity and specificity were 97 and 98%, respectively. LA was simple and reliable in the rapid identification of streptococci from blood cultures and appeared to be preferable to IF. When LA is used, the group D reagent should not be used, and all samples reacting with the group C reagent should be tested by a modified rapid bile solubility test to exclude S. pneumoniae.