A comparison of four-sample slope–intercept and single-sample 51Cr-EDTA glomerular filtration rate measurements

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The aim of this study was to verify, with a large dataset of 1394 51Cr-EDTA glomerular filtration rate (GFR) studies, the equivalence of slope–intercept and single-sample GFR. Raw data from 1394 patient studies were used to calculate four-sample slope–intercept GFR in addition to four individual single-sample GFR values (blood samples taken at 90, 150, 210 and 270 min after injection). The percentage differences between the four-sample slope–intercept and each of the single-sample GFR values were calculated, to identify the optimum single-sample time point. Having identified the optimum time point, the percentage difference between the slope–intercept and optimal single-sample GFR was calculated across a range of GFR values to investigate whether there was a GFR value below which the two methodologies cannot be considered equivalent. It was found that the lowest percentage difference between slope–intercept and single-sample GFR was for the third blood sample, taken at 210 min after injection. The median percentage difference was 2.5% and only 6.9% of patient studies had a percentage difference greater than 10%. Above a GFR value of 30 ml/min/1.73 m2, the median percentage difference between the slope–intercept and optimal single-sample GFR values was below 10%, and so it was concluded that, above this value, the two techniques are sufficiently equivalent. This study supports the recommendation of performing single-sample GFR measurements for GFRs greater than 30 ml/min/1.73 m2.

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