Tailoring the sampling time of single-sample GFR measurement according to expected renal function: a multisite audit
Abstract BackgroundThe 2018 BNMS Glomerular Filtration Rate (GFR) guidelines recommend a single-sample technique with the sampling time dictated by the expected renal function, but this is not known with any accuracy before the test. We aimed to assess whether the sampling regime suggested in the guidelines is optimal, and determine the expected error in GFR result if the sample time is chosen incorrectly. We can then infer the degree of flexibility in the sampling regime.Methods Data from 8946 patients referred for GFR assessment at 6 different hospitals for a variety of indications were reviewed. The difference between the single-sample (Fleming) GFR result at each sample time and the slope-intercept GFR result at each hospital was calculated. A second dataset of 775 studies from one hospital with nine samples collected from 5 minutes to 8 hours post injection was analysed to provide a reference GFR to which the single sample results were compared.Results Recommended single-sample times have been revised: for estimated GFR above 80 ml/min/1.73m2 a 2 hour sample is recommended, giving mean difference from slope-intercept GFR of -2.08 ml/min/1.73m2 (1333 GFR tests included). Between 30 and 80 ml/min/1.73m2 a 4 hour sample is recommended, giving a 1.95 ml/min/1.73m2 mean difference (2057 GFR tests included). The standard deviation of the differences is 3.50 ml/min/1.73m2 at 2 hours and 2.56 ml/min/1.73m2 at 4 hours for GFR results in the recommended range. It is 5.81 ml/min/1.73m2 at 2 hours and 5.70 ml/min/1.73m2 at 4 hours for GFR results outside the recommended range. ConclusionThe results of this multisite study demonstrate a reassuringly wide range of sample times for an acceptably accurate single-sample GFR result. Modified recommended single-sample times have been proposed in line with the results, and the reported errors for both sample times can be used for error analysis of a mistimed sample.