How good is the slope of the second exponential for estimating 51Cr-EDTA renal clearance?

The aim of this study was to test the reproducibility of the second exponential of the plasma disappearance curve of 51Cr-EDTA and to compare it with the single slope-intercept method. The test was performed twice among 24 young healthy volunteers at an interval of 8 days and under similar physiological conditions. After the intravenous injection of 3.7 MBq of 51Cr-EDTA, four blood samples were taken between 1.5 and 4 h and combinations of two, three or all four of these were used to calculate the slope of the plasma clearance curve. The clearances rates were calculated using these slopes and the corresponding intercepts of the slopes with the y-axis. The standard deviation of the differences between the first and the second measurements was between 8 and 10% for the different clearance estimates, and between 11 and 17% for the different slope estimates, reflecting the wider variability of the slope estimate. Using the first series of measurements, the change of timing for blood sampling introduced a wider variability for the slope estimate than for the clearance estimate.