Tag Archives: sputum

“A paradigm shift…”

At the moment most antibiotics are initiated without waiting for the microbiological result, if they are thought to be clinically necessary.

Quite right too.

This is so called “empirical therapy”.

There is a good reason for this. Traditionally, microbiology tests have neither been good enough nor fast enough to make the antibiotic prescription dependent on the result.

Take for example the patient who sees his GP with a productive cough and fevers. The GP is not going to say to the patient “Let’s just wait a few days until we get the sputum culture result back. By the way there is a good chance it will be negative even if you do happen to have a pneumonia. And on the flipside, if it does grow something it might just represent the bacteria in your throat.”

No chance…, the GP will simply prescribe an antibiotic based on the most likely pathogens, and also the local antibiotic susceptibility patterns.

Along the same lines, the GP is not going to say to the patient a couple of days later. “Your sputum sample has come back negative, so let’s stop your antibiotic.” Sputum cultures are nowhere near sensitive enough to allow this approach.

On the very odd occasion, the treatment will actually change as a consequence of the microbiology result, if there happens to be an unusual or resistant organism.

And sputa are only one sample type. Ear swabs, peri-anal swabs, & ulcer swabs probably have even less impact on patient management…

In fact for the vast majority of  samples (probably > 95%) that get processed by the microbiology laboratory, the impact on patient management is rather small indeed.

However change is coming on to the horizon. The newer microbiological tests, and in particular the polymerase chain reaction (PCR) assays, are both fast enough and sensitive enough to start genuinely impacting on patient management right at the time of prescribing.

Take the following potential examples:

  • Macrolide antibiotics could be witheld in a patient with suspected whooping cough until the Bordetella pertussis PCR is back.
  • Patients with urethral discharge and suspected gonorrhoea would only be treated if the Neisseria gonorrhoeae PCR result comes back positive.
  • Patients with meningo-encephalitis could have acyclovir to cover HSV, dependent on the CSF viral panel result.
  • Legionella cover in a patient with moderate to severe community acquired pneumonia could be dependent on the result of the Legionella PCR in a sputum sample.

These are all tests which, if performed quickly enough, can significantly reduce the amount of antibiotics given to the tested cohort, so they all have potential to play a big part in any antimicrobial stewardship program.

So we need to get such assays into our microbiology laboratories, whatever it takes.

Microbiology matters, but we need to ensure that we utilise new tests and technology to make it matter even more…

Michael

 

“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