COMPARISON OF SINGLE-INJECTION MULTISAMPLE RENAL CLEARANCE METHODS WITH AND WITHOUT URINE COLLECTION

Single-injection renal clearance methods based on plasma clearance alo ne, without urine collection, are sometimes met with skepticism. They require data extrapolation to infinite time, which is hard to justify a priori. It has been asserted that they are less accurate for rapidly cleared tubular agents than for slowly cleared glomerular filtration rate agents. In this study, we compare urine-based and urine-free meth ods for the tubular agents Tc-99m-MAG3 and I-131-OIH. Methods: In 18 p atients, dual-tracer plasma data were obtained from 4 to 90 min after injection (nine samples). Urine was also collected for 90 min (in two voidings). The urine counts were corrected for residual bladder activi ty by pre- and postvoid dual-channel gamma camera images. Results: Whe n comparing the two methods of clearance calculations, the difference between urine-based and urine-free measurements was 1 +/- 5 ml/min for Tc-99m-MAG3 and 23 +/- 8 for I-131-OIH (mean +/- s.e. of the mean). F or Tc-99m-MAG3, the regression line did not differ significantly from the line of identity. The correlation coefficient was 0.94 for both ag ents. Conclusion: Urine collection is not necessary to measure renal c learance, even for the rapidly cleared tubular agents, except at low c learance levels (when the small absolute error corresponds to a large percentage error).