Evaluation of new streptococcal latex grouping kit

To evaluate a new streptococcal latex grouping kit (Shield Diagnostics Ltd) and compare it against an established latex agglutination method (Streptex; Wellcome Diagnostics).

Two hundred and forty seven strains of streptococci and enterococci were tested with each kit by one operator and according to the manufacturer's instructions. Strains failing to group or giving discordant results were identified to species level.

Two discrepant grouping results were observed and 13 non-beta haemolytic streptococci failed to group with either product. The Shield kit successfully identified 232 isolates at 15 minutes of enzyme extraction incubation compared with 224 and 233 on short (15 minutes) and long (1 hour) incubations, respectively, for Streptex (p > 0.23 for both comparisons). On short incubation only, the Shield kit detected significantly more strains of Enterococcus faecium (p = 0.007). The reaction strengths were similar for both kits (p > 0.16). No cross-reactions were observed but the Streptex kit produced significantly fewer tests with visible granularity (p < 0.003).

Although the Shield product appeared to detect group D antigen more readily, overall no important differences in performance were observed. Prospective users of the new method should first become familiar with its characteristics.