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Both 99m Tc-DTPA and 51 Cr-EDTA are widely used to determine glomerular filtration rate (GFR), but few direct comparative studies exist. The shortage of 51 Cr-EDTA makes a direct comparison highly relevant. The aim of the study was to investigate if there is any clinically relevant difference between plasma clearance of 99m Tc-DTPA and 51 Cr-EDTA. Patients ≥18 years of age referred for routine GFR measurement by 51 Cr-EDTA were prospectively enrolled. The two tracers (10 MBq 99m Tc-DTPA (CaNa 3 -DTPA) and 2.5 MBq 51 Cr-EDTA) were intravenously injected at time zero. A standard 4-sample technique was applied with samples collected at 180, 200, 220 and 240 min, if the estimated GFR (eGFR) was ≥30 mL/min. A comparison of single-sample GFR based on the 200 min sample was also conducted. Fifty-six patients were enrolled in the study. All patients had an estimated GFR >30 mL/min/1.73 m 2 . No patients suffered from ascites or significant oedema. The mean 51 Cr-EDTA plasma clearance was 82 mL/min (range 16-226). The plasma clearances determined by the two methods were highly correlated ( r  = 0.993). The plasma clearance was significantly higher when measured by 99m Tc-DTPA than by 51 Cr-EDTA ( p  = 0.01), but the numerical difference was minimal (mean difference 1.4 mL/min; 95% limits of agreement (LOA) -6.6 to 9.4). The difference between the two methods was independent of the level of renal function. Similar results were found for one-sample GFR. No clinically relevant differences were found between the plasma clearance of 99m Tc-DTPA and that of 51 Cr-EDTA. Therefore, 99m Tc-DTPA can replace 51 Cr-EDTA when needed.

Background: Total plasma clearance of (51)Cr-EDTA, Cl, is widely used as a measure of GFR. Commonly, only the final part of the plasma concentration curve is measured, and a one-pool clearance (slope-intercept clearance), Cl(1), is computed. Empirically determined second-order polynomials of the general form Cl = b x Cl(1) + c x Cl(1)(2) are usually used to estimate Cl from a measured Cl(1). However, theoretical considerations indicate that such corrections underestimate Cl at high values., Aims: To derive an analytically correct relationship between Cl and Cl(1) and determine the parameters involved for children and adults., Material and Methods: Cl was determined in 149 subjects (M/F/children: 71/46/32) from a complete plasma concentration curve followed for 4-5 h after injection of (51)Cr-EDTA (range of clearance: 8-183 mL/min/1.73 m(2)). Plasma volume, PV and the "missing" area under the plasma fraction curve, a (minutes), not used for determination of Cl(1), were measured., Results: The true relationship between Cl and Cl(1) is given by Cl = Cl(1)/(1 + f x Cl(1)), where f = a/PV. For men, women and children alike, the equation f = 0.0032 x BSA(-1.3) was applicable (BSA = body surface area in m(2)). Estimation errors on clearance were within +/-8% for adults and +/-13% for children (95% limits of agreement)., Conclusions: The true relationship between Cl and Cl(1) of (51)Cr-EDTA is given, resulting in a common correction equation applicable for children and adults. The new equation has better mathematical behaviour than quadratic equations on very high values of clearance and takes into account dependence on body size.

Background: The one-pool or slope-intercept technique is widely used when determining total (51)Cr-EDTA plasma clearance (Cl). The one-pool clearance (Cl(1)), which always exceeds Cl, has mostly been corrected to Cl by multiplication by a constant factor = 0.80, suggested by Chantler (CH(0.80)), or by using a second-order polynomial originally proposed by Brøchner-Mortensen (BM) and later recommended by the British Nuclear Medicine Society (BM(BNMS)). Theoretical considerations indicate that the CH correction gives a systematic overestimate of Cl, whereas the BM correction may underestimate Cl at high values., Objective: To assess the accuracy of Cl as estimated from Cl (1) corrected either by CH(0.80) or by second-order polynomials., Material and Methods: Cl(ref) was determined in 149 subjects (M/F/children: 71/46/32) from a complete plasma curve followed for 4-5 h after injection of (51)Cr-EDTA (range of Cl(ref) : 8-183 mL/min/1.73 m(2)). Cl(est) was determined from Cl(1) subsequently corrected by CH(0.80) and four second-order polynomials., Results: Using CH(0.80) correction, Cl(est) underestimated Cl(ref) (by a maximum of 20%) at Cl(ref) values less than about 100 mL/min/1.73 m(2) in children and 130 mL/min/1.73 m(2) in adults. At higher clearance levels, Cl(ref) was increasingly overestimated. Taking the BM(BNMS) correction as representative of second-order polynomials, Cl(est) increasingly underestimated Cl(ref) at high levels, the error being 10% at a Cl(ref) value of about 175 mL/min/1.73 m(2)., Conclusions: We suggest that the tested correction equations are replaced by the given common correction equation based on the "true" relationship between Cl(1) and Cl thoroughly described in part I of this study.