Mannitol clearance for the determination of glomerular filtration rate-a validation against clearance of Cr-EDTA.

We studied the agreement between plasma clearance of mannitol and the reference method, plasma clearance of Cr-EDTA in outpatients with normal to moderately impaired renal function. Forty-one patients with a serum creatinine <200 μmol l entered the study. Cr-EDTA clearance was measured with the standard bolus injection technique and glomerular filtration rate (GFR) was calculated by the single-sample method described by Jacobsson. Mannitol, 0·25 g kg body weight (150 mg ml ), was infused for 4-14 min and blood samples taken at 1-, 2-, 3- and 4-h (n = 24) or 2-, 3-, 3·5- and 4-h after infusion (n = 17). Mannitol in serum was measured by an enzymatic method. Plasma clearance for mannitol and its apparent volume of distribution (Vd) were calculated according to Brøchner-Mortensen. Mean plasma clearance (±SD) for Cr-EDTA was 59·7 ± 18·8 ml min . The mean plasma clearance for mannitol ranged between 57·0 ± 20·1 and 61·1 ± 16·7 ml min and Vd was 21·3 ± 6·2% per kg b.w. The between-method bias ranged between -0·23 and 2·73 ml min , the percentage error between 26·7 and 39·5% and the limits of agreement between -14·3/17·2 and -25·3/19·9 ml min . The best agreement was seen when three- or four-sample measurements of plasma mannitol were obtained and when sampling started 60 min after injection. Furthermore, accuracy of plasma clearance determinations was 88-96% (P30) and 41-63% (P10) and was highest when three- or four-sample measurements of plasma mannitol were obtained, including the first hour after the bolus dose. We conclude that there is a good agreement between plasma clearances of mannitol and Cr-EDTA for the assessment of GFR.