Chronic renal failure is widely underestimated. Serum creatinine-based formulas proposed for extensive evaluation of chronic renal disease are utilized for a population generally under 70. However in elderly pts serum creatinine levels are falsely low due to decreased skeletal muscle index (SMI ) , so in old subjects it is difficult to assess renal function on the basis of creatinine levels. Aim of our study was to compare glomerular filtration rate (GFR) values estimated according to 51Cr-EDTA Brochner monocompartimental slope-intercept method (considered as a gold standard) with several, different creatinine based formulas to estimate the best approach to evaluate renal function.
We assessed 12 pts over 70 (7 M) measuring GFR by Brochner monoexponential method and then comparing this figure with five serum creatinine-based formulas : BIS-1 , Cokroft-Gault, CKD-EPI , MDRD and with 24h urine collection creatinine clearance. All pts were over 70 and all were sarcopenic (< 7.26 Kg/m2 in men and < 5.5 Kg/m2 in women); pts age was 81.1 ± 6.5 years , serum creatinine figures were 1.15 ± 0.52 , SMI was 5.06 ± 1.17. We administered e.v. 5 MBq of 51Cr-EDTA and then took 3 plasma samples at 180’ , 210’ and 240’ or, if presumptive clearance was < 30 ml/min, at 180’ , 240’ and 300’.
In our pts 51Cr-EDTA clearance was 43.84 ± 19.26, clearance figures were for BIS-1 formula 50.01 ± 14.54, for MDRD 61.96 ±21.83 , for Cockroft 34.62 ± 14.8 , for CHD-EPI 57.74 ± 20.65 , for 24h creatinine clearance 39.5 ± 7.07. Best linear regression fit was obtained between 51Cr-EDTA GFR and BIS-1 formula (r2= 0.85). The r2 values between 51Cr-EDTA method and CKD-EPI (Chronic Kidney Disease – Epidemiology Collaboration) method, MDRD (Modification of Diet in Renal Disease) method and Cokroft-Gault method resulted 0.83; 0.82 and 0.70 respectively. BIS-1 formula was proposed Schaffner ES (Ann Internal Medicine, 2010) in over 70 population as the more accurate in old pts compared with iohexol GFR estimate.
In clinical work-up 51Cr-EDTA is the most reliable method to assess renal function mainly in elderly pts in which sarcopenia is often present; however 51Cr-EDTA clearance study is time consuming and requires a nuclear medicine facility. In our experience BIS-1 formula gave results closest to 51Cr-EDTA method and therefore we recommend BIS-1 formula to calculate GFR in elderly, sarcopenic patients. However, also CKD-EPI and MDRD offer results very close to 51Cr-EDTA and are, therefore suitable.