I spent a lot of my medical student days working part-time in the bookmakers, where managing odds and probability underpin the industry. Mathematics was my favourite subject at school, and within that field, probability was my favourite sub-topic.

Now that I am a clinical microbiologist, I can see the similarities. Clinical microbiologists get involved in a lot of the early antimicrobial stewardship decisions for patients based on preliminary/interim microbiology results. This is an area where you definitely need to weigh up the percentages/odds.

For example if you get a positive blood culture with gram positive cocci on the Gram, what will be the probability that this will turn out to be a Staph aureus or even an MRSA? If you are awaiting susceptibilities on an E.coli bacteraemia, what will be the chances it will be an ESBL? These are decisions which have to be made when advising optimal antibiotic therapy. So many factors need to be considered, almost sub-consciously, when calculating these odds.

Of course, with all decisions that involve probability, you will never win (get it right) all of the time. This is where it is critically important to take into account the potential consequences of getting it wrong, i.e. how sick is the patient? how much “reserves” do they have, what level of care are they currently in? In a “game” which involves probabilities, it is careful and prudent management of these probabilities which is key.

The other field of probability that clinical microbiologists must have understanding of are pre- and post- test probability, and positive and negative predictive value. These are key concepts in determining the validity of any test result the laboratory produces.

Playing the odds is an integral part of being a clinical microbiologist. It may even be part of the reason I was attracted to the specialty in the first place. I can even pretend I am still working at the bookies…

Michael