Interpretation of blood cultures yielding Staphylococcus aureus

Forty-eight patients with blood cultures positive for Staphylococcus aureus were classified according to clinical criteria in three groups: "definite", "possible", and "doubtful" septicemia. Using traditional blood culture sets with two bottles (thioglycollate and tryptic soy broths), we found that patients with "definite" septicemia always showed more than one positive bottle per day if more than one set was drawn, that the mean detection time was 1.7 days, and that 95% of the first positive bottles and 92% of all positive bottles grew within two days of incubation. Patients with "doubtful" septicemia were more often (88%) positive in one bottle only, the mean detection time for all bottles was 3.7 days, and only 35% of the first positive bottles and 33% of all positive bottles yielded growth within two days. "Possible" cases took a position between these two extremes but tended more towards the "doubtful" cases. The implications of these findings for the interpretation of blood cultures with S. aureus are discussed.