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

The extent of cross-reactivity between Streptococcus pneumoniae and two commercial streptococcal latex grouping reagents was examined. Cross-reactivity with the group C latex reagent was demonstrated in 91% (20 of 22) of blood cultures tested directly and in 97% (73 of 75) of clinical isolates of various serotypes tested after enzymatic extraction. This is important in the clinical laboratory because it may lead to erroneous identification of organisms present in blood cultures.