“Sorting out Staphylococcus aureus in Blood Cultures”

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.


2 thoughts on ““Sorting out Staphylococcus aureus in Blood Cultures”

  1. I’m not sure Michael. I would be very uncomfortable ever calling S. aureus a contaminant. I would treat a S. aureus in one of two bottles (nearly) every time … what leads you to believe that half of these are contaminants?

    1. It is anecdotal but based on 5years of contributing data to the ANZCOSS(Australian New Zealand Cooperative on Outcomes from Staphylococcal Sepsis) where we looked closely at all SABs(several hundred) and tallied the clinical data. Unfortunately I did not keep a database of which ones were considered contaminants, which in retrospect would have been very useful. The actual numbers where just one bottle out of the set was positive were actually relatively small so the confidence margins were large, but I do believe the figure was somewhere in this ballpark. Given that 15-30% of people are colonised with Staph aureus it is not surprising that a few contaminants crop up from time to time. Nevertheless I agree with your sentiments. You cannot be too careful. Thanks for commenting, Michael

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