Just because a stool sample turns up at your microbiology laboratory, it doesn’t mean you have to test it… This is old style microbiology reasoning, testing for everything in the hope that you will find something!
There are many different microbiology tests that one can do on a stool sample. Here is a sample list of what is offered at the lab I work at:
- PCR for common bacterial pathogens, e.g. salmonella, campylobacter, shigella, VTEC, yersinia.
- Culture for more opportunistic bacterial pathogens such as Aeromonas
- EIA for cryptosporidium and giardia
- GDH/PCR for C. difficile toxin
- Faecal concentration and trichrome stain for ova, cysts and parasites
- Immunochromatographic assay for rotavirus
- Multiplex PCR for other enteric viruses (e.g. noro, astro, sapo)
- Faecal antigen test for H. pylori.
With appropriate clinical details present, we can then choose objectively from the list above which tests are appropriate to perform for a specific sample.
However, without clinical details, it would be utterly unreasonable for the lab to do all of these tests, and without clinical details there is no way of deciding which tests we should be doing.
Yet so many microbiology labs still take this approach. Receive a stool sample and test it for something! This is blindfold microbiology.
Extending this philosophy further, clinical details of “diarrhoea” doesn’t really cut the mustard either. That is to some extent stating the obvious!
Fit healthy adults who present with a short history of diarrhoea in general do not require laboratory testing. Personally I get 2 or 3 episodes of loose stools every year. I am sure the rest of the world has a similar experience! I do not need laboratory testing. So clinical details simply of “diarrhoea” or “loose stools” is insufficient to justify testing. There needs to be more than that…
The lab I work at will only test stool samples if one of the following applies, even when clinical details of “diarrhoea” or something similar is on the form:
- Something to indicate an illness on the more severe end of the spectrum, such as prolonged diarrhoea, bloody diarrhoea, hospitalised, systemic symptoms, etc.
- Or something that suggests there might be a public health issue, e.g. food handler, group meal, overseas travel, farm worker, etc.
“Carte blanche” approaches to enteric microbiology are hideously costly, and also give rise to quality issues such as overdiagnosis and overtreatment.
If you test every stool sample you receive for putative pathogens such as Blastocystis hominis or Dientamoeba fragilis, you are going to end up overdiagnosing and overtreating a whole heap of people. Don’t go there!
By taking a considered and objective approach to microbiology testing of stool samples you can dramatically reduce the amount of testing that you perform, and increase the quality of results at the same time.