Key concepts in clinical epidemiology: reporting on the accuracy of continuous tests

Many clinical diagnostic tests, such as the joint fluid white blood cell count, produce results on a continuous scale, rather than a mere positive or negative. The accuracy of such tests is often reported as a positive and negative likelihood ratio at each of several potential cutoff points (e.g., ≥25,000/μL vs. not, ≥50,000/μL vs. not; ≥100,000/μL vs. not). This Key Concepts article reviews the definition of a likelihood ratio and explains why the practice of dichotomizing the test is problematic. Instead, it proposes that such continuous scales be divided into multiple intervals (e.g., 0–25,000, >25,000–50,000, >50,000–100,000, >100,000) and each interval be given its own likelihood ratio. This practice not only aligns with clinical common sense and practice but also enables a more accurate estimate of the updated risk of disease, given a pre-test risk.