When I worked in a microbiology laboratory in Glasgow we hardly ever reported antibiotic susceptibilities on Staphylococcus aureus when cultured from sputum. Now I am working in New Zealand, and we almost always do…
So who is right and who is wrong?
The problem is that Staphylococcus aureus in the sputum can mean several different things:
- It may simply represent oropharyngeal contamination of the sample.
- On the flipside it may signify a severe necrotising pneumonia in an immunocompromised or post-influenza patient.
- And thirdly, we know that Staphylococcus aureus can colonise or occasionally cause “low level” infection in the architectually damaged lung (e.g. cystic fibrosis, COPD, bronchiectasis)
So in summary, it can mean absolutely nothing, or it could signify a life threatening illness…
The clinical context and sample quality are clearly key here to working out what is going on. However, in actual practice, the sputum sample often arrives into the laboratory without any clinical details, so we are processing blind.
“Just do your stuff, and give us the result…”
So how should we manage this problem from a laboratory point of view?
Here are a few potential solutions:
- Reject sputum samples for culture where the Gram stain shows lots of squamous epithelial cells representative of oropharyngeal contamination. (A lot of labs have now adopted this approach, including my own)
- Report susceptibilities routinely on Staphylococcus aureus from hospitalised (& cystic fibrosis) patients only.
- Add a comment saying that close clinical correlation is required in the interpretation of this result & susceptibilities will be available on request only.
or all of the above…
The clinical context is always important for the laboratory to issue a correct report. However, for sputum samples growing Staphylococcus aureus, it is absolutely critical.
Or one could be even stricter, and just say, “no clinical details, no test”…