Rapid identification of Staphylococcus aureus from blood culture bottles by a classic 2-hour tube coagulase test

Abstract

The rapid, reliable identification of Staphylococcus aureus from positive blood cultures provides important information. While multiple methodologies for detection of S. aureus from blood culture broths exist, none is satisfactory. Immunologic tests have shown varied sensitivities, the thermonuclease test, while sensitive, is not practical for routine use, and probe tests are expensive. Few studies have addressed using the tube coagulase test (TCT). This study compared two immunologic methods, the Staph Latex kit (Remel Laboratories) and the Staphaurex kit (Wellcome Diagnostics), with a rabbit plasma TCT (Difco Laboratories) to identify S. aureus within 2 h directly from blood culture broths and pelleted supernatants from BACTEC (Johnston Laboratories) bottles. One hundred twelve unique clinical blood culture isolates consistent with a Gram stain for staphylococci and 68 negative blood culture bottles seeded with a variety of gram-positive organisms were evaluated. Sensitivity and specificity among clinical specimens for the 2-h TCT were 79.5 and 100%, respectively. Sensitivities for the immunologic methods were 12.8 and 10.2% for the Staphaurex and Remel Staph Latex, respectively, and specificities for both were 100%. These results contradict previously reported results for both immunologic and TCT methods and dictate that a specific as well as a sensitive method be employed. The 2-h TCT was found to be a cost-effective, reliable, and rapid method for identifying S. aureus from positive blood cultures.