Critical Review of Method Comparison Studies for the Evaluation of Estimating Glomerular Filtration Rate Equations

Background: Bland and Altman have published their method of differences in 1986 in the Lancet to draw their method to the attention of medical professionals. Bland and Altman always have pointed out that their method of differences is designed to detect bias between methods. Their statistical methodology has found its way among clinical chemists, but it has been criticized in 2002 that a high proportion of clinical chemists misinterpreted or misused the methods of differences. It seems that the Nephrology community is falling into the same traps as clinical chemists if they use the method of differences to compare estimating glomerular filtration rate (eGFR) equations with the direct measurement of GFR (mGFR). Methods: Using simulated data and a real life example, we demonstrate how the method of differences should be used appropriately. Findings: The main points of criticism are that proportional bias is not analysed correctly and that the calculated 95% limits of agreement should be compared to predefined clinically acceptable limits. Interpretation: There is no need to introduce ‘alternative’ statistics like absolute mean difference, accuracy within 30%, etc when the BlandAltman methodology is correctly applied. The goal of method comparison is to demonstrate how close the new method is to the (g)old method. The final decision to accept or reject the eGFR-equation is a clinical decision, not a statistical decision. The real problem with currently available eGFR-equations is that none of these equations are close enough to the measured GFR so that the eGFR-equation can replace the measured GFR.