RESUMO
BACKGROUND: The 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 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 6328 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 777 studies from one hospital with nine samples collected from 5 min to 8 h 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 an expected GFR above 90 ml/min/1.73m2 a 2-h sample is recommended; between 50 and 90 ml/min/1.73m2 a 3-h sample is recommended; and between 30 and 50 ml/min/1.73m2 a 4-h sample is recommended. Root mean square error in single-sample GFR result compared with slope-intercept can be kept less than or equal to 3.30 ml/min/1.73m2 by following these recommendations. CONCLUSION: The 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 a lookup table has been produced of rms errors across the full range of GFR results for the three sample times which can be used for error reporting of a mistimed sample.
RESUMO
AIM: We aimed to investigate the accuracy of a single-sample glomerular filtration rate (SS-GFR) technique with a sample taken at 24 h post-injection for patients with GFR lower than 25 mL/min/1.73 m2. A comparison with the results from same-day slope-intercept GFR (SI-GFR) was also performed. METHODS: Data from patients referred for GFR assessment to inform the management of chronic kidney disease at the Royal Free Hospital were reviewed. Four-sample SI-GFR calculation with samples at 2-, 4-, 6-, and 24-h post-injection was taken as the reference measurement to which the Gref and Karp SS-GFR (24-h sample) and same-day SI-GFR (2- and 4-h samples) were compared. The effect of protein binding on GFR accuracy was modelled. RESULTS: A total of 43 GFR examinations with reference GFR less than 25 mL/min/1.73 m2 were included in the analysis. Bland-Altman analysis gave mean differences of 0.4 mL/min/1.73 m2 (95% confidence interval: 0-0.7) for SS-GFR (24 h) and 3.0 mL/min/1.73 m2 (95% confidence interval: 1.9-4.2) for same-day SI-GFR. 95% limits of agreement were -2.0 to 2.8 mL/min/1.73 m2 for SS-GFR (24 h) and -4.0 to 10.1 mL/min/1.73 m2 for same-day SI-GFR. CONCLUSIONS: SS-GFR with a 24-h sample is more accurate than same-day SI-GFR in patients with GFR less than 25 mL/min/1.73 m2. Using SS-GFR with a 24-h sample in routine clinical practice will result in clinically insignificant differences in GFR result compared with the reference technique, whereas a same-day SI-GFR measurement could cause large inaccuracies.
