Comparison of glomerular filtration rate measurements using inulin, 51CrEDTA, and a phosphate infusion technique

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1. 1. Measurements of glomerular filtration rate (GFR) using inulin, 51CrEDTA, and a phosphate infusion technique were compared in order to assess their relative accuracy. 2. 2. GFR was determined from phosphate data in a series of 66 infusions in 50 subjects. Comparison with simultaneous inulin clearance (180 measurements during 38 infusions) gave a ratio of GFR derived from phosphate data (GFR p) /mean inulin clearance of 1.00 ± 0.03 (S.E.). Comparison with simultaneous 51CrEDTA clearance (189 measurements in 33 infusions) gave a ratio of 51CrEDTA clearance/GFR p of 0.94 ± 0.03 (S.E.). 3. 3. Separate direct comparison of renal inulin clearance with 51CrEDTA clearance in another series of 65 measurements from 15 infusions gave a ratio of 51CrEDTA clearance/inulin clearance of 0.96 ± 0.02 (S.E.). 4. 4. The method of determining GFR without urine collection was also used to compare inulin with 51CrEDTA. In 13 infusions this urineless GFR ratio of 51CrEDTA/ inulin was 0.92 ± 0.04 (S.E.). The urineless GFR gave significantly higher absolute values than mean standard clearances, in the ratio of 1.07 for both inulin and 51CrEDTA; the discrepancy was probably due to incomplete equilibration resulting from purposely shortened infusion times. 5. 5. It was concluded that an infusion technique based on the measurement of phosphate alone gives an accurate measure of GFR, that GFR is consistently underestimated by the use of 51CrEDTA, and that the method without urine collection overestimates renal clearance when infusion times are shorter than 4 h.