special subjects
Understanding Medical Tests and Test Results
Continuous Tests
 can be calculated for single or multiple cutoff points. The table Effect of Changing the Cutoff Point of the WBC Count in Patients Suspected of Having Appendicitis illustrates the effect of changing the cutoff point of the WBC count in patients suspected of having appendicitis. . For example, the table Using WBC Count Groups to Determine Likelihood Ratio of Bacteremia in Febrile Children* illustrates the relationship between WBC count and bacteremia in febrile children. Because the LR is the probability of a given result in patients with disease divided by the probability of that result in patients without the disease, the LR for each grouping of WBC count is the probability of bacteremia in that group divided by the probability of no bacteremia. Grouping continuous variables allows for much greater use of the test result than when a single cutoff point is established. Using Bayesian analyses, the LRs in the table Using WBC Count Groups to Determine Likelihood Ratio of Bacteremia in Febrile Children* can be used to calculate the post-test probability. For continuous test results, if an ROC curve is known, calculations as shown in the Using WBC Count Groups to Determine Likelihood Ratio of Bacteremia in Febrile Children* do not have to be done; LRs can be found for various points over the range of results using the slope of the ROC curve at the desired point. 