Measurement of glomerular filtration rate by free vitamin B12 clearance

We determined the vitamin B12 clearance using an ultrafiltration technique and assessed whether the clearance of this vitamin B12 could be used to estimate the glomerular filtration rate (GFR). Fourteen subjects (5 had renal disease, 7 had diabetes mellitus, one had liver cirrhosis and one had cholelithiasis) divided into two groups were studied (group 1, 5 patients without vitamin B12 preloading; group 2, 9 patients with vitamin B12 preloading). Vitamin B12 clearance was significantly correlated with inulin clearance (r = 0.81, p < 0.001) in group 1; group 2 showed an even better correlation (r = 0.94, p < 0.001) with the presaturated vitamin B12 binding protein. In group 2, the mean inulin and vitamin B12 clearance values do not differ significantly (40.3 +/- 13.6 vs 48.2 +/- 17.2 ml/min), but there was a significant difference between mean inulin and creatinine clearance (40.3 +/- 13.6 vs 64.9 +/- 19.9 ml/min, p < 0.05). In conclusion, vitamin B12 clearance appears to be a more reliable method of estimating GFR than creatinine clearance.