Here’s my subjective and anecdotal take on interpreting Staphylococcus aureus isolates in blood cultures.
Overall, when Staph aureus isolated from blood cultures, it is clinically significant about 90-95% of time, and a contaminant in the remainder.
To break it down a bit further:
- When both bottles are positive for Staph aureus, then likely to be significant 99% of the time. You would be very brave to call a Staph aureus a skin contaminant when isolated from both bottles (However I have seen this happen on several occasions, and probably erroneously). It should only ever be done with the full consensus of the clinicians and repeat set/s of blood cultures.
- When only one bottle in a set of 2 is positive for Staph aureus, it is probably significant about 50% of the time. Again, close clinical correlation and repeat blood cultures are advised.
- When it is just one bottle (ie a paediatric bottle) that is received into the lab, then a Staph aureus isolate probably has a significance rate somewhere in between these two figures above.
To falsely call a Staphylococcus aureus in a blood culture a contaminant could be potentially fatal for the patient, so best to err on the side of caution. A good stance to take is: Staphylococcus aureus bacteraemia is clinically significant until proven otherwise.