Six- versus Four-Hour Iohexol Clearance for Glomerular Filtration Rate Measurement in Living Kidney Donor Candidates

Accurate assessment of glomerular filtration rate (GFR) in potential living kidney donors is necessary to ensure adequate kidney function for donation. In the United States, GFR is most commonly approximated using timed urinary creatinine clearance, but this method is subject to inaccuracy based on collection challenges and non-GFR creatinine clearance via tubular secretion.^1,2 Measured GFR (mGFR) based on clearance of an exogenous marker provides a more accurate assessment of GFR but is resource intensive.² Shorter testing protocols may improve the feasibility of mGFR assessment in clinical practice. We sought to assess the differences in mGFR results from 6-hour versus 4-hour iohexol clearance testing.

We identified all living kidney donor candidates who underwent mGFR testing at our center from 01/01/2023 – 08/31/2024, a period during which 6-hour iohexol clearance was assessed. At our center, potential donors are referred for mGFR testing if either creatinine-based estimated GFR (eGFR) or 24-hour timed creatinine clearance are <90 mL/min/1.73m². For testing, 1.5 mL of cold iohexol was administered via intravenous injection, and the syringe was weighed before and after injection to determine the exact delivered iohexol dose. Serum iohexol concentration was measured via high-performance liquid chromatography in blood sampled 2, 3, 4, and 6 hours after injection. GFR was subsequently calculated from iohexol clearance using the Bröchner-Mortensen correction, and then adjusted for body surface area (BSA, calculated using the Gehan and George formula where BSA (m²) = 0.0235 × Weight(kg)^0.51456 × Height(cm)^0.42246).^3,4 In 7 cases, the 2-hour sample was drawn from an angiocatheter contaminated with iohexol; in these cases, mGFR was calculated after excluding the 2-hour sample for both clinical purposes and for this analysis. We calculated mGFR based on the 6-hour iohexol clearance as performed clinically (“mGFR-6h”), then recalculated mGFR based only on samples obtained during the first 4 hours of testing (“mGFR-4h”). eGFR using the combination of creatinine and cystatin C was calculated using the CKD-EPI-2021 equation. Pearson correlation coefficient was calculated to determine the correlation between patient-level mGFR-6h and mGFR-4h. Analyses were conducted using Stata/MP 17 (StataCorp, TX). This study was approved the institutional review board of Columbia University Medical Center.

Among 128 mGFR tests conducted during the study period, 3 were excluded due to not having a 6-hour sample drawn. Among 125 included tests, median age was 53 years (IQR 45–61), 42 (34%) were male, and median BMI was 26.7 kg/m² (IQR 23.8–30.5) with median BSA 1.89 (IQR 1.72–2.07). Median eGFR was 89 ml/min/1.73m² (IQR 81–98).

Median mGFR-6h was 80.5 ml/min/1.73m² (IQR 73.4–90.4) and median mGFR-4h was 80.7 ml/min/1.73m² (IQR 72.8–90.0), corresponding to a median difference of −0.03 ml/min/1.73m² (IQR −0.81 – 1.05) and a median absolute difference of 0.98 ml/min/1.73m² (IQR 0.44–1.93) (Figure 1A). 114 (91%) had an absolute difference between median mGFR-6h and median mGFR-4h of <5 ml/min/1.73m², and 121 tests (97%) had absolute difference within 10% of mGFR-6h. Correlation between mGFR-6h and mGFR-4h was r=0.9726 (p<0.001, Figure 1B).

Figure 1. Measured glomerular filtrate rate (mGFR) assessed using 6-hour versus 4-hour cold iohexol clearance among living kidney donor candidates.

(A) Difference between 6-hour mGFR and 4-hour mGFR, where the red line indicates the median difference of −0.03 ml/min/1.73m². (B) Scatterplot of 6-hour mGFR versus 4-hour mGFR, where the blue line with shading indicated the line of best fit with 95% confidence interval and the scored black line is the line of concordance.

These data indicate that shortening iohexol plasma clearance testing from 6 hours to 4 hours provides near identical mGFR results for living kidney donor candidates. Given the simplicity of plasma iohexol clearance testing compared to urinary iothalamate clearance or radioisotope clearance, short testing protocols are feasible to be completed in clinic while donor candidates are undergoing pre-donation evaluations (e.g., with blood draws between sequential appointments) and can reduce the patient burden and cost of testing compared to 6-hour protocols. Future studies should explore the use of even shorter mGFR protocols that may reduce the time burden on living donor candidates. A large, international study of 5,344 mGFR tests across a wide range of GFR values found excellent performance of single-sample iohexol testing compared to multi-sample testing in individuals with ranges similar to those seen in donor candidates.⁵ Single-sample testing at 3 hours after injection yielded mGFR results within 10% of multi-sample test results in 96% of individuals with GFR 60–90 and 98% of those with GFR>90.⁵ Risks of this approach may include overreliance of a single sample in cases of measurement error, although this risk appears low based on the study’s results. Clinicians must also still consider that, despite being less prone to error than eGFR or timed creatinine clearance, any mGFR testing remains subject to error related to assay accuracy and nonideal behavior of iohexol.² Further, these findings are not necessarily applicable to mGFR testing in other clinical settings, as patients with low GFR likely require later sampling for improved accuracy.²