Plasma clearances of inulin (Cin), 99mTc-DTPA (CDTPA) and urographie contrast media (CCM) were determined simultaneously in 31 patients with varying levels of renal function evaluated in the setting of affiliated cardiac and renal transplantation programs. Cin and CDTPA were calculated from the ratio of simultaneously measured plasma concentration and urine excretion rate of (U x v — P)• CCM was derived from x-ray fluorescence measurement of plasma iodine (PI) content following intravenous injection of 50 ml of nonionic, low-osmolar contrast media (180 mg I/ml). Urine collections were not required for CCM determinations. No adverse reactions attributable to CM occurred in any patient, and follow-up serum-creatinine values did not differ significantly from prestudy levels. CCM determined from the rate of decline in PI between 3 hr and 4 hr following administration o f contrast media (“slope-intercept” technique) [Ccm—SI] correlated closely with corresponding levels of C ln (r = .86, P<0.0001), and CDTPA (r = 0.89, P<0.0001). Mean CCM-SI/Cin and CCM-SI/CDTPA ratios for the entire study cohort were 1.09 ± 0.06 and 1.08 ± 0.06, respectively. CCM-SI determinations also correlated well with CCM levels derived from a single measurement of PI (“single sample” technique) made at 3 hr following injection of contrast media (r = 0.94, PcO.OOOl). Both CCM-SI and CCM determined by the “single sample” method (CCM-3°SS) tended to overestimate Cin and CDTPA, however, when the latter were <20 ml/min/1.73 m2 (mean CCM-SI/Cin and Ccm-3°SS/Cin in ratios 1.36±0.14 and 1.95±1.0, respectively. Reproducibility was evaluated by paired comparison of 3-hr vs. 4-hr “single sample” CCM determinations (r = 0.99, P<0.0001). In addition, analysis o f the variation in iodine content between duplicate specimens obtained at each o f these time intervals revealed a mean ratio o f 1.0±0.01 (P = NS vs. identity). Contrast clearance determination utilizing the slopeintercept method is accurate, safe, pragmatic, and more precise than serum-creatinine and endogenous-creatinine clearance for measurement of renal function in clinical transplantation.
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