Tag Archives: sputum

“Thresholds”

 

I have noticed while authorising sputum culture reports, that some people have high thresholds for working up suspect colonies from plate cultures, only proceeding when the potential pathogen is dominant amongst the upper respiratory tract flora that is also inevitably present.

Other people have very low thresholds for working up suspect colonies, painstakingly trying to pick out potential pathogens even when there only a few of the same colony type present in the milieu.

Depending on what your threshold is, it clearly has the potential to produce a different result for the clinician.

But who is right and who is wrong? Is it that sometimes we just try a bit too hard?

My gut instinct is that a few colonies of a respiratory pathogen (e.g. Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis) nestling amongst a mass of mixed upper respiratory tract flora is unlikely to be significant, but I am not aware of any literature that supports such an assertion.

I tend not to get too excited by sputum cultures as a rule… Clinicians don’t hang around waiting for the patient to firstly produce a sputum sample, and then wait another 2 or 3 days whilst the microbiology laboratory processes it. They treat the patient empirically, according to guidelines that are hopefully formulated by laboratory data outlining the expected pathogens and antibiograms in the local area.

Only on rare occasions does a sputum culture result actually change patient management.

If “sputum culture” were a new immunoassay, and therefore subject to FDA approval before being released to the market, it wouldn’t have a prayer of being accepted. The sub-optimal sensitivity and specificity (even with prior filtering of samples using macroscopic appearance and Gram stain) would simply not cut it from a regulatory point of view.

So we will continue to grapple with the vagaries of sputum culture, but I suspect it will be around for many years yet.

But we should not lose any sleep over it, whatever your threshold is…

Michael

“Staphylococcus aureus in sputum samples. A reporting conundrum.”

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”…

Michael

“Aspergillus fumigatus isolation in sputum samples: A marker for structural lung disease?”

Aspergillus fumigatus is an aesthetically pleasing micro-organism. 

 Aspergillus

It can also cause a variety of diseases as listed:

  • Invasive Aspergillosis
  • Allergic Bronchopulmonary Aspergillosis
  • Aspergilloma
  • Hypersensitivity Pneumonitis
  • Atopic Asthma

In most of the clinical diseases above, the isolation, or lack thereof, of Aspergillus fumigatus from sputum bears little relation to the likelihood of the disease being present. I have only seen a handful of each disease entity during my career. However I have seen hundreds of Aspergillus fumigatus isolates from sputum cultures, most of which I am sure are just “colonising” the patient.

So in most cases, what does isolation of Aspergillus from sputum samples mean?

It is my belief that in most cases for Aspergillus to colonise the respiratory tract, there needs to be the presence of “architectural/structural lung disease”. This could be COPD, bronchiectasis, bronchial carcinoma or even cystic fibrosis. There may be the occasional aspergillus isolate in a sputum culture which has resulted from airborne contamination during sampling. However serial isolates of Aspergillus from sputa in a patient with no history of structural lung disease should certainly prompt investigations into this possibility.

There have certainly been studies looking at the prevalence of Aspergillus in patients with structural lung disease, but little that I could find that looks at the specificity of sputum Aspergillus isolates in predicting structural lung disease.

In conclusion I suspect that Aspergillus fumigatus colonisation is quite a strong marker for structural lung disease and would certainly be an area worth studying further, if only I could find the time…..

Michael

p.s. I have added a powerpoint presentation on Automation in the Microbiology laboratory, a presentation co-written by myself and Sean Munroe, a microbiology scientist working at Waikato Hospital, Hamilton, New Zealand.