Variability in diagnostic accuracy can be estimated using simple population weighting.

Objective: Diagnostic accuracy of a quantitative diagnostic test at a given numeric cutoff is dependent on the type of population (e.g., chronic, referrals, or screening) under investigation. Simple weighted averages calculated from a single study sample may be used to assess variability in accuracy in different types of populations.
Study Design and Setting: We evaluated the accuracy of the 4-variable Modification of Diet in Renal Disease (MDRD) Study equation as a diagnostic test to separate stage 1 and 2 chronic kidney disease (>or=60 mL/min per 1.73 m(2)) from stage 3-5 (<60 mL/min per 1.73 m(2) requiring treatment to prevent progression) in a sample of 850 patients referred for determination of glomerular filtration rate (GFR). Using population weighting, we also estimated the accuracy of the MDRD equation when the GFR distribution typically found in screening situations was mimicked.
Results: Estimated diagnostic accuracy of the MDRD equation varied substantially for different population types (sensitivity range 82%-97%, specificity 67%-93%; figures include the original MDRD study).
Conclusions: Reports of diagnostic accuracy should include estimates of the variability of diagnostic accuracy, using different real or tentative population distributions. Population weighting is a useful tool for this purpose.