Accuracy of next-day single-sample measurement for low glomerular filtration rate and comparison with same-day slope-intercept glomerular filtration rate

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 m(2). 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 m(2) were included in the analysis. Bland-Altman analysis gave mean differences of 0.4 mL/min/1.73 m(2) (95% confidence interval: 0-0.7) for SS-GFR (24 h) and 3.0 mL/min/1.73 m(2) (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 m(2) for SS-GFR (24 h) and -4.0 to 10.1 mL/min/1.73 m(2) 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 m(2). 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.