Coagulase testing compared with commercial kits for routinely identifying Staphylococcus aureus

Abstract

Five commercial Staphylococcus aureus identification kits - Staphaurex (Wellcome), Staphylase (Oxoid), Staphyslide (bioMerieux), Biostaph (Medlabs) and Bacto Latex (Difco) - were evaluated for the routine identification of S. aureus from primary plates in the routine microbiology laboratory. Comparison was made with two methods of tube coagulase testing and five slide methods for detecting clumping factor (slide coagulase testing). Performances were assessed for two groups of organisms, staphylococcal species alone and a combined staphylococcal and non-staphylococcal species group. The effects of growth on selective media and storage of isolates at room temperature and 4°C were investigated. Selective media cannot be recommended, nor can storage of isolates before testing. Ranked according to efficiency values with the combined staphylococcal and non-staphylococcal species group, the kits and coagulase methods performed as follows (the figures in parentheses are the efficiency values for the staphylococcal group alone): tube coagulase reference method 100% (100%), tube coagulase SJH method 99% (99%), Staphaurex 94% (97%), Staphylase 93% (96%), slide coagulase method No 4 93% (94%), slide coagulase method No 5 93% (93%), Bacto Latex 92% (95%), Staphyslide 92% (95%), and Biostaph 87% (91%). It is concluded that a commercial S. aureus identification kit should not replace tube coagulase testing for the routine identification of the organism from primary plates and that, even the kits with the best performances, have little advantage over a good slide coagulase test method.