Using diagnostic tests
Nicola Cooper offers a guide on how to use a diagnostic test
How many times have you seen doctors use a diagnostic test to rule out a disease or heard someone say on the ward round, “This patient must have x disease because of x test?” As a doctor in training you may get the impression that a test gives a diagnosis. The purpose of this article is to give you a more accurate perspective on diagnosis and the use of diagnostic tests (see box 1).
Doctors have to make decisions without definitive information all the time because there is no such thing as a perfect test. Imagine a good test, which has 90% sensitivity and 90% specificity (see box 2). Ninety per cent of patients with the disease will have an abnormal result and 90% of patients who do not have the disease will have a normal result. But this leaves 10% of patients with the disease with a normal result