ABSTRACTS PRESENTADOS AL 11th INTERNATIONAL SYMPOSIUM "RADIONUCLIDES IN NEPHRO-UROLOGY", REALIZADO EN MONTEREY, CALIFORNIA, 13-17 MAYO 2001.

M. Rehling, S.S. Nielsen and L.E. Nielsen.
· Dept. of Nuclear Medicine, Aarhus University Hospital, Skejby Hospital, Aarhus, Denmark.

Five methods have been published for the measurement of plasma clearance with the single-injection single-sample technique. One method uses renal clearence of MAG3 as reference (Bubeck), four use a multiple-sample plasma clearence method with blood sampling from 5 to 60-100 min p.i. We speculate whether differences between the single-sample methods at least in part could be due to the choice of reference clearance method.

Aim: To study the importance of reference MAG3 clearance method for the measurement of the single-injection single-sample MAG3 plasma clearence.

Material and Methods: Forthy five patients with GFR 10-132 ml/min were included. After a single-injection of MAG3, clearance was assessed from 1) 8 blood samples 5-90 min p.i. (Clp5-90), 2) 16 blood samples 2-300 min p.i. (Clp 2-300), 3 ) renal clearence 2-300 min p.i. (Clr).

Results: Mean ratio (sem): Clp 5-90/Clr=1.28(0.03), Clp2-300/Clr=1.09(0.02), and Clp5-90/Clp2-300=1.17(0.01).

Discussion: Renal clearance of MAG3 was overestimated by 28% by plasma clearance assessed from blood samples 5-90 min p.i. because of a great overestimation of the final slope of the plasma curve. There was a much better agreement when blood sampling was prolonged until 300 min p.i. . In the upper range of renal function it has been shown earlier that higher clearance values are obtained with Russells single-sample method compared with Bubecks method. The reason seems to be that Russell has used a plasma clearcnce reference method based on blood samples 5-90 min p.i., while Bubeck has used renal clearance as reference.

Conclusion: Plasma clearance of MAG3 based on blood samples 5-90 min p.i. overestimates renal clearance. Later blood samples are needed to obtain an accurate result. Presumably single-sample plasma clearance methods based on blood samples 5-90 min p.i. overestimate renal clearance correspondingly.