Progression-related bias in the monitoring of kidney function in patients with diabetes and chronic kidney disease

The Cockcroft and Gault (CG) and Modification of Diet in Renal Disease (MDRD) equations underestimate the glomerular filtration rate (GFR) decline in diabetes. Do this decline and the albumin excretion rate (AER) influence their validity ? In 161 diabetic patients, isotopically determined GFR (i-GFR) (51Cr-EDTA) was compared with estimated GFR (e-GFR) by the CG, MDRD, and the new Mayo Clinic Quadratic (MCQ) equations. We searched for a relation between the error in e-GFR and the AER. An influence of the AER outcome on the e-GFR decline was evaluated in 63 subjects followed up over 3 years. The MDRD and the MCQ were more precise and accurate than the CG, but they were biased. The error increased with AER for the CG (r = 0.25, P = .001) and the MDRD (r = 0.20, P = .009), but not for the MCQ. For the 63 patients followed up, the e-GFR declines by the 3 estimations were related to the initial AER, whereas no relation with arterial blood pressure, hemoglobin A1C, hemoglobin, and blood lipids emerged. The MCQ declines were more pronounced: −10.5% ± 8.9% in the macroalbuminuric group (P < .05 vs both microalbuminuric [−2.6% ± 10.1%] and normoalbuminuric [−0.1% ± 6.6%] groups), and were related to the outcome of the AER (r = 0.33, P < .05). As chronic kidney disease progresses in diabetes, the declining GFR and rising AER influence the estimation of GFR by the CG and MDRD equations, underestimating the GFR decline and the benefit of reducing the AER. The less affected MCQ evidences a slower e-GFR decline with AER control.