The use of labelled vitamin B12 in the measurement of glomerular filtration rate.

1. Two preparations of radioactive vitamin B(12) labelled respectively with (57)Co and (58)Co have been employed in the estimation of the glomerular filtration rate (G.F.R.) in dogs. The intravenous injection of these preparations at an interval of 20 min resulted in a changing ratio of their activities in plasma and urine and so permitted a relation in time to be established between the urine and the plasma from which it was filtered.2. When urine and plasma were sampled at short intervals an effective renal delay or transit time could be derived from their isotope ratio curves and used as a correction in the clearance equation. This allowed serial clearances of either isotope to be calculated despite a falling plasma concentration.3. This method was applied to the study of the G.F.R. in fourteen dogs using short urine collections of 2 or 4 min for periods up to 80 min. The results under stable conditions and with deliberately induced changes in the G.F.R. are illustrated and discussed.4. Application of the technique in four patients undergoing differential renal function studies gave results comparable with those in the experimental work.5. The plasma binding of administered vitamin B(12) in serial blood samples in dogs and man was investigated.