Prior probability

Shaping a differential diagnosis is a process of categorizing the probabilities of various diseases associated with the patient history and exam. The probability that a patient has the disease before any diagnostic testing is done is known as the prior probability of disease.

Diagnostic testing increases or decreases the likelihood that a person has the disease in question. The likelihood that person has the disease after diagnostic testing is known as the posterior probability.

Diagnostic testing occurs along a continuum of likelihoods when the prior probability is lower than the treatment threshold but higher than the testing threshold. If the prior probability is above the treatment threshold, one treats the patient without further testing.

If the prior probability is below the treatment threshold, further testing is required, the results of which either move one away from the treatment threshold if they are negative or closer or over the treatment threshold if they are positive.

A series of tests may be necessary to move the posterior probability that a patient has the disease to the point of treatment. As an illustration, if one operates on a person only if one is 70% sure that he or she has the disease, but after the history and physical one is only 40% sure, then one needs to do more testing to move the likelihood above 70%.

There is no need to perform tests when one is already over the treatment threshold (although these are often done for academic reasons) or if the test will not provide enough evidence to move one over the treatment threshold if no other testing is linked or available.

For example, if a febrile patient has tender anterior cervical anterior cervical adenopathy with an exudative pharynx and the absence of cough by history, he or she should be treated for bacterial pharyngitis without further testing.

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