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, rathe r, have a range of possible results (very negative to very positive). If th e pretest probability of disease is high, the test result that prompts trea tment should be any value that is even mildly positive. If the pretest prob ability of disease is low, the test result needed to justify treatment shou ld be very positive. Simple decision rules that fix the cutpoint separating positive from negative test results do not take into account the individua l patient's pretest probability of disease. Allowing the cutpoint to change with the pretest probability of disease increases the value of the test. T his is primarily an issue when the pretest probability of disease varies wi dely 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 patien t-specific risk. This tutorial demonstrates how the value of a diagnostic t est depends on the ability to vary the cutpoint, using as an example the wh ite blood cell count in febrile children at risk for bacteremia.