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.