Suppose that based on symptoms a patient has, his doctor is 60% certain that the

Question

Suppose that based on symptoms a patient has, his doctor is 60% certain that the patient has a particular disease. If doctor's suspicions would be overwhelming, say at least 85%, then he would recommend a surgery. Under these circumstances, the doctor opts for quite an invasive and expensive procedure, which unfortunately is not 100% reliable. In particular, the test can show positive even if the patient does not have the disease (false positive), because of his diabetes. Chances of this is 30%. On the other hand, the test can show negative if the patient does have the disease (false negative) in 10% of all cases. Question is, in the event the test shows positive, how much higher the prior estimate of 60% should increase to make the test worth rendering. Can we accurately predict the results of this before running the test in order to see if a positive test will elevate the prior from 60% to 85% or higher?

Explanation / Answer

results may help make a diagnosis in symptomatic patients (diagnostic testing) or identify occult disease in asymptomatic patients (screening). However, test results may interfere with clinical decision making if the test poorly discriminates between patients with and without disease, if the result is inconsistent with the clinical picture, or if the test result is improperly integrated into the clinical context. Laboratory tests are imperfect and may mistakenly identify some healthy people as diseased (a false-positive result) or may mistakenly identify some affected people as disease-free (a false-negative result). A test's ability to correctly include or exclude disease depends on how likely a person is to have a disease (prior probability) and on the test's intrinsic operating characteristics.

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