Rapid identification of Staphylococcus aureus in blood cultures by means of the direct tube coagulase test

Abstract

Rapid differentiation of Staphylococcus spp. strains grown in bood cultures is the basis for early and appropriate treatment of S. aureus infections.

Objective: To evaluate the usefulness of the direct tube coagulase test done from blood culture vials with presumptive growth of Staphylococci.

Methods: Direct tube coagulase test was carried out if only clusters of gram-positive cocci were observed in the blood culture vials; interpretation was done four hours later and results were compared with those obtained with colonies from subcultures.

Results: 1.518 direct tube coagulase tests were carried out; of them, 411 (27.1%) were positive and 1.107 (72.9%), negative. Out of the 446 strains of S. aureus that were isolated, 410 had positive direct tube coagulase test (sensitivity 91.9%). An additional strain of S. aureus was positive in the direct test but negative in the subculture (specificity 99.9%). Predictive values were 99.7% (positive) and 96.7% (negative).

Conclusions: Our results confirm the usefulness of the direct tube coagulase test done from blood cultures to opportunely differentiate staphylococci grown in blood cultures. Information so obtained may be used to decide the beginning of empirical treatment.