A Quantitative Study of the Decision Threshold for the Diagnosis of Infectious Mononucleosis

Background: Ordering a serologic test for infectious mononucleosis (IM) in all young patients with sore throat is costly and impractical. The test threshold to determine when to order a diagnostic test for IM based on the patient’s symptoms has not been previously studied. Objective: To determine the test threshold for IM in the management of patients with sore throat. Design and Setting: Online surveys were sent to a convenience sample of US primary care clinicians regarding their decision making about whether or not to order a test for IM in a patient with sore throat. Method: 7 clinical vignettes were created, each with a different combinations of symptoms and signs. The probability of IM for each vignette was estimated by the investigator based on the number of symptoms present to generate a plausible range of disease probabilities. Clinicians were then asked to decide whether to test or not test for IM, and mixed-effect logistic regression was used to determine the test threshold for IM where half of physicians chose to test and half chose not to test. Results: A total of 117 clinicians provided responses for a total of 819 clinical vignettes. The overall test threshold for IM as estimated using the logistic regression was 9.5% (95% CI: 8.2% to 10.9%). The test threshold for clinicians practicing greater than 10 years was significantly higher than for those practicing less or equal to 10 years (10.5% vs 7.3%, P = .02). No significant differences between specialties and practice sites were found with respect to the test threshold. Conclusion: This study identified a test threshold for IM of approximately 10% based on realistic clinical vignettes. This threshold was stable regarding the clinician’s specialty and practice sites and could be used in the development of a clinical prediction rule to determine the cutoff for low- versus high-risk groups.