“The exception and the majority”

When to only test for the majority and when to look for the exception dictates so much of our testing in microbiology.

For example:

  • Is it reasonable to put up plates to specifically look for Neisseria gonorrhoeae in every conjunctival swab?
  • Is it reasonable to give prolonged incubations to all sputa from immunocompromised patients in the hope of finding Nocardia?
  • Is it reasonable to exclude a Vancomycin Resistant Enterococcus in every urinary enterococcal isolate?
  • Is it reasonable to look for Arcanobacterium haemolyticum in all throat swabs?
  • Is it reasonable to look for Giardia lamblia in all stool samples?


You will all have your own opinions on these questions and there are no right or wrong answers.

What influences whether we only test for the majority, or look also for the exception depends on many factors, the cost of the testing, the severity of the disease, the local epidemiology, the trust that the users have in the laboratory, and many more… 

We cannot test for everything in the laboratory, we cannot cover for every exception. A lot of our testing policy is dictated by whether we let the exception rule the majority, or whether we test only for the majority at the expense of the exception.

As I have said, there is no right or wrong answer, but we must always be able to provide a rationale for what we do and don’t test for based on the factors above.


p.s. I have added a quick tutorial on Metronidazole to the website. Click here for tutorials.

6 thoughts on ““The exception and the majority”

  1. Very interesting post. At the moment I am thinking about whether or not we should do trich culture for every female HVS between 13-59y or only where PMN and/or clinical details indicate.

    1. I know what I would do Arlo, given half a chance! Seriously though a retrospective audit looking at trichomonas prevalences within and without these cohorts might help your decision.

  2. We started adding a chocolate plate to all urine cultures from the ER a long time ago. We have found MANY unsuspected positives for Neisseria gonorrhoeae by doing this. It is a State reportable and is followed up throughout the area.

    1. Thanks for the comment Frank. Interesting idea and one I don’t think I have come across before. One potential downside is that it may infer to users that urine is an optimal sample for Neisseria gonorrhoeae culture, which we know it isn’t. However as I have said, there are no right or wrong answers…

  3. Most of our ER physicians are ordering GC/Chlamydia NAA tests whenever they suspect it, however, we are finding a lot of positives on women who present with UTI and TNTC wbc’s on the microscopic urine. Cultures are either mixed or pure cultures of GC.

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