Tailoring the sampling time of single-sample GFR measurement according to expected renal function: a multisite audit

Abstract

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.

Variability of estimated glomerular filtration rate and 99mTc-DTPA glomerular filtration rate: implications for a single time-point sampling regime

“…The analysis was repeated for the four ranges, suggested by McMeekin et al (2022) as opposed to the five ranges specified in the BNMS 2018 guidelines [ 1 , 7 ]. It would be reasonable to assume decreasing the number of ranges may improve the sensitivity in individual windows as these will be wider; however, this was not observed.…”

“…Of those incorrectly predicted most of these samples would have been predicted to an adjacent sampling time-point with six studies (2.1%) predicted incorrectly to two sampling time-points away. The increase in uncertainty resulting from incorrect sample time is relatively small [ 7 ]. Additionally, the adoption of BNMS 2018 guidelines has been slow in part due to the practical complexities of the number of different time-points.…”

“…The RMS err for different sample times is shown in Fig. 3 of McMeekin et al (2022) where they demonstrated a ‘wide range of sample times for an acceptably accurate single-sample GFR result’ [ 7 ].…”

“…For inclusion studies required a sample time within 20 min of specified sampling time and eGFR measurement within 28 days of mGFR. One study was excluded as the serum creatinine measurement was not recorded, six studies were excluded as the sample time was more than 20 min outside the time specified in the protocol and one study was excluded as the eGFR measurement had been performed 31 days prior to the GFR study, compliant with local protocol which specifies 1 month but outside the 28-day limit applied here in line with other work in the field [ 7 , 8 ]. Studies with large errors associated with the estimation of body surface area (BSA) (amputees) or volume of distribution (ascites/oedema) or both were also excluded ( n = 17).…”

“…BNMS 2018 guidelines recommend five different sample time-points, dependent on patient-expected GFR [1]. A recent multi-centre study by McMeekin et al (2022) calculated the error on the single-sample mGFR measurement as compared to the slope-intercept mGFR measurement and based on the minimum error recommended that sample time should be chosen from four ranges instead of five [7].…”