“Medicine is a science of uncertainty and an art of probability.” William Osler

This article is an extension of a previous article on sensitivity, specificity, and positive predictive value, this time looking at pre- and post-test probability.

Take the following hypothetical scenario:

A 15 yr old teenager and an 85 yr old Rest Home resident both present to their doctor with a sore throat and cervical lymphadenopathy.

An Epstein Barr Virus (EBV) screening test (with sensitivity and specificity both at 97%) is positive in both patients.

Without knowing any other information, what is the likelihood of each person having Infectious Mononucleosis/Glandular Fever due to EBV?

Let’s look at it before the EBV test is performed. The prevalence of glandular fever in 15 year olds with sore throats is many times greater than in 85 year olds. Thus the chance of the 15 yr old having glandular fever is much, much higher than the 85 yr old. (This is called pre-test probability).

After the test result is known, the post-test probability of glandular fever is extremely high in the teenager, but still relatively low in the 85 yr old, as the very low prevalence in this age cohort will lower the positive predictive value of the test. Therefore the chances of the EBV result being a false positive in the 85 year old is relatively high.

In conclusion, exactly the same result in different patients needs to be interpreted differently.

So what is the lesson from this?

I suspect that a lot of laboratory users don’t really think enough about pre and post-test probability when they see the laboratory result. They may well take the result at face value and diagnose the patient on the basis of it. (I have seen this happen many many times…)

It is our job in the laboratory to convince the requestors that on the basis of the factors described above, the results are not always perfect, however much we would like them to be……

Michael

p.s. I have added a quick powerpoint on the basics of Norovirus Infection to the website.