I know it is difficult to believe but I was doing a little bit of background reading recently on Monospot tests (looking for heterophile antibodies to Epstein Barr Virus). On reading a guideline I came across this statement…
“The presence of heterophile antibodies in a symptomatic adolescent or young adult has a sensitivity of approximately 90%, and specificity of almost 100% for glandular fever.”
Now the question I have is what is wrong with this statement?
The answer is that sensitivity and specificity are functions of the test itself. Different tests for the same disease from different manufacturers may have different sensitivity and specificity.
However once you start applying the test to population cohorts such as symptomatic adolescents, then you need to start talking in terms of positive and negative predictive value.
….and the paradox is when you use a test such a Monospot, with a sensitivity of approximately 80%, in a high prevalence population such as symptomatic adolescents, then your negative predictive value will be relatively lower than in a low prevalence cohort, as there will be a significant amount of people who will test negative who actually have the disease (false negatives).
Sometimes we need to think about the science behind the statements in the guideline and make sure it makes sense in our heads.
Don’t believe everything you read, (especially when it is written by me!)
For a really nice presentation on sensitivity, specificity, PPV etc click here (5-10 minute read)