If you could get a multiplex PCR result back to you within 2 hours that told you your hot, grumpy, 2 month old patient did not have bacteraemia, would you discontinue antibiotics?

How sure would you need to be of that result – 95% certain? 98% certain? 99.5% certain?

What – in diagnostic analysis speak – would be your ‘threshold’?

The deciding of a threshold can be emotional, or rational, and both. The rational part of the idea relies on an equation:

Probability of wrong decision * consequence vs. probability of right decision * advantages

Rationality – based on a good understanding of some of the attributes of the diagnostic test in question – provides the probabilities. The ’emotional’ part comes in when assessing what weight the consequences and the advantages both have. (Now there are rational approaches to coming up with these – utility values they get called – but they’re still emotional at heart.)

Sometimes the consequence is so emotionally overwhelming that there is no degree of ‘chance’ that is allowed to be acceptable.

But what about you? What value would you place? If the test was 99% correct – would that be enough? Comments welcome.

• Archi

(You may recall this is the same maths that runs the decision
in treatment too.)