What White Blood Cell Count Should Prompt Antibiotic Treatment in a Febrile Child? Tutorial on the Importance of Disease Likelihood to the Interpretation of Diagnostic Tests

Most diagnostic tests are not dichotomous (negative or positive) but, rather, have a range of possible results (very negative to very positive). If the pretest probability of disease is high, the test result that prompts treatment should be any value that is even mildly positive. If the pretest probability of disease is low, the test result needed to justify treatment should be very positive. Simple decision rules that fix the cutpoint separating positive from negative test results do not take into account the individual patient’s pretest probability of disease. Allowing the cutpoint to change with the pretest probability of disease increases the value of the test. This is primarily an issue when the pretest probability of disease varies widely between patients and depends on characteristics that are not measured by the test. It remains an issue for decision rules based on multiple test results if these rules fail to account for important determinants of patient-specific risk. This tutorial demonstrates how the value of a diagnostic test depends on the ability to vary the cutpoint, using as an example the white blood cell count in febrile children at risk for bacteremia.