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

Abstract

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 Streptococcusfaecalis 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.

Evaluation of several commercial biochemical and immunologic methods for rapid identification of gram-positive cocci directly from blood cultures

“…Several immunologic group D reagents were evaluated for reliability in replacing the bile esculin test for identifying group D streptococci. Our results with Streptex are in contrast to the results of a study reported by Shales et al (14) in which only 61.5% of isolates were correctly identified with this reagent. In their study, they tested blood culture broth rather than the concentrated pellet.…”

“…Since bacteremias caused by these organisms can cause life-threatening infections, prompt detection and identification are essential for proper patient management. Several rapid commercial immunologic and biochemical reagents have been developed for isolate identification which can also be applied to the identification of gram-positive cocci directly from blood cultures (4,14,16,17). A systematic study of these systems applied in a dichotomous scheme has not been reported.…”

Latex agglutination testing directly from throat swabs for rapid detection of beta-hemolytic streptococci from Lancefield serogroup C

“…There are several possible explanations for the few positive rapid tests which were not confirmed by culture. Antigenic cross-reactions between group C BHS and S. pneumoniae and Klebsiella pneumoniae have been reported by using latex agglutination methods with blood culture and skin swab specimens (9,12,17). False-positive results of the trivalent color test for group C organisms have also been reported for individuals with blood group A (13).…”

Cross-reaction between Streptococcus pneumoniae and group C streptococcal latex reagent

“…The original Lancefield precipitation method for the serogrouping of beta-hemolytic streptococci has been largely superseded by more rapid and sensitive methods, such as latex agglutination. Latex agglutination has also been used for the rapid detection of bacteria or antigens in blood cultures (12)(13)(14), body fluids (1,5), and swabs taken from various sites of infection (2,3,7,10). Although previous studies indicate that reasonably reliable results can be obtained, the potential for antigenic cross-reactions among different bacteria has not been fully appreciated.…”