The estimation of glomerular filtration rate in an Australian and New Zealand cohort

Estimated GFR: time for a critical appraisal

Since 1957, over 70 equations based on creatinine and/or cystatin C levels have been developed to estimate glomerular filtration rate (GFR). However, whether these equations accurately reflect renal function is debated. In this Perspectives article, we discuss >70 studies that compared estimated GFR (eGFR) with measured GFR (mGFR), involving ~40,000 renal transplant recipients and patients with chronic kidney disease (CKD), type 2 diabetes mellitus or polycystic kidney disease. Their results show that eGFR often differed from mGFR by ±30% or more, that eGFR values incorrectly staged CKD in 30-60% of patients, and that eGFR and mGFR gave different rates of GFR decline. Errors were unpredictable, and comparable for equations based on creatinine and/or cystatin C. We argue, therefore, that the persistence of these errors (despite intensive research) suggests that the problem lies with using creatinine and/or cystatin C as markers of renal function, rather than with the mathematical methods used for GFR estimation.

Modification of Diet in Renal Disease versus Chronic Kidney Disease Epidemiology Collaboration equation to estimate glomerular filtration rate in obese patients

BACKGROUND

Obesity is a recognized risk factor for both the development and progression of chronic kidney disease (CKD). Accurate estimation of glomerular filtration rate (GFR) is thus important in these patients. We tested the performances of two creatinine-based GFR estimates, the Modification of Diet in Renal Disease (MDRD) and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations, in an obese population.

METHODS

Patients with body mass index (BMI) > 30 kg/m(2) were included. The reference method for measured GFR (mGFR) was (51)Cr-EDTA (single-injection method, two blood samples at 120 and 240 min). Both indexed and non-indexed results were considered. Serum creatinine was measured using the IDMS-traceable compensated Jaffe method. Mean bias (eGFR-mGFR), precision (SD around the bias) and accuracy within 30% (percentage of estimations within 30% of mGFR) were calculated for both equations.

RESULTS

The population included 366 patients (185 women) from two different areas. Mean age was 55 ± 14 years, and mean BMI was 36 ± 7 kg/m(2). Mean mGFR was 56 ± 26 mL/min/1.73 m(2) (71 ± 35 mL/min without indexation). In the total population, mean bias was +1.9 ± 14.3 and +4.6 ± 14.7 mL/min/1.73 m(2) (P < 0.05), and accuracy 30% was 80 and 76% for the MDRD and CKD-EPI equations (P < 0.05), respectively. In patients with mGFR > 60 mL/min/1.73 m(2), mean bias was +4.6 ± 18.4 and +9.3 ± 17.2 mL/min/1.73 m(2) (P < 0.05), and accuracy 30% was 81 and 79% (NS) for the MDRD and CKD-EPI equations, respectively.

CONCLUSIONS

The CKD-EPI equation did not outperform the MDRD study equation in this population of obese patients.