A diagnosis and treatment plan is drawn up following history-taking and clinical examination at the bedside or in the consulting room. These elements of every consultation are vital to good practice and save a good deal of time, money, problems and uncertainty, provided they are used properly. Each element of history-taking, clinical examination and diagnostic testing has a particular diagnostic value, changing the prior probability of a diagnosis into a posterior probability. If the prior probability is low it will often be decided not to collect further information. For example, we do not ask a patient presenting with knee problems whether they also happen to have a headache; we do not carry out a digital rectal examination on a patient with epilepsy for no reason; and if a patient has had non-radiating back pain for many years with no neurological symptoms or loss of function it is better not to request an MRI of the lumbar spine. This all changes, of course, if diagnosing the condition could have major consequences for the patient.
