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Both 99mTc-DTPA and 51Cr-EDTA are widely used to determine glomerular filtration rate (GFR), but few direct comparative studies exist. The shortage of 51Cr-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 99mTc-DTPA and 51Cr-EDTA. Patients ≥18 years of age referred for routine GFR measurement by 51Cr-EDTA were prospectively enrolled. The two tracers (10 MBq 99mTc-DTPA (CaNa3-DTPA) and 2.5 MBq 51Cr-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 m2. No patients suffered from ascites or significant oedema. The mean 51Cr-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 99mTc-DTPA than by 51Cr-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 99mTc-DTPA and that of 51Cr-EDTA. Therefore, 99mTc-DTPA can replace 51Cr-EDTA when needed. [ABSTRACT FROM AUTHOR]

BACKGROUND: In GFR measurements with radiotracers, there is evidence that a two-compartment model is unable to describe the full plasma curve, including early time points, but analyses generally focus on two-compartment models. AIMS: To analyze both the mammillary and catenary three-compartment model and to determine empirical relations between model constants and the overall GFR and ECV (extra-cellular volume). MATERIAL AND METHODS: Mathematical analysis of the three-compartment model. Full-curve patient data from 32 adults and 7 children were used to relate model parameters to GFR and ECV. RESULTS: Model volumes were found to be roughly proportional to ECV. In both models, the central (plasma) volume was V1 = 0.24 x ECV and elimination rate from V1 was k10 = 4.2 x GFR/ECV. In the mammillary model, the two parallel volumes were V2 = 0.28 x ECV, V3 = 0.48 x ECV, and intercompartmental clearances were Cl12 [mL/min] = 0.0058 x ECV [mL], Cl13 = 0.042 x ECV. In the catenary model, the serial volumes were V2 = 0.60 x ECV, V3 = 0.16 x ECV, with clearances Cl12 = 0.048 x ECV, Cl23 = 0.0036 x ECV. CONCLUSION: Insight into the three-compartment model was achieved, and empirical relations to ECV and GFR/ECV were determined. [ABSTRACT FROM AUTHOR]

Background . The one-pool or slope-intercept technique is widely used when determining total 51Cr-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 (CH0.80), or by using a second-order polynomial originally proposed by Brøchner-Mortensen (BM) and later recommended by the British Nuclear Medicine Society (BMBNMS). 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 CH0.80 or by second-order polynomials. Material and methods . Clref was determined in 149 subjects (M/F/children: 71/46/32) from a complete plasma curve followed for 4-5 h after injection of 51Cr-EDTA (range of Clref : 8-183 mL/min/1.73 m2). Clest was determined from Cl 1 subsequently corrected by CH0.80 and four second-order polynomials. Results . Using CH0.80 correction, Clest underestimated Clref (by a maximum of 20 %) at Clref values less than about 100 mL/min/1.73 m2 in children and 130 mL/min/1.73 m2 in adults. At higher clearance levels, Clref was increasingly overestimated. Taking the BMBNMS correction as representative of second-order polynomials, Clest increasingly underestimated Clref at high levels, the error being 10 % at a Clref value of about 175 mL/min/1.73 m2. 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. [ABSTRACT FROM AUTHOR]

Background. Total plasma clearance of 51Cr-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⋅Cl 1+c⋅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 51Cr-EDTA (range of clearance: 8-183 mL/min/1.73 m2). 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⋅Cl 1), where f = a/PV. For men, women and children alike, the equation f = 0.0032⋅BSA -1.3 was applicable (BSA = body surface area in m2). 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 51Cr-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. [ABSTRACT FROM AUTHOR]