Serum creatinine measurement as a predictor for single sample GFR using 99mTc-DTPA

Aim/Introduction: Radionuclide measured Glomerular Filtration Rate (mGFR) assessment using 99mTc-DTPA (Diethylenetriamine Pentaacetic Acid) is widely used in Nuclear Medicine for patients pre-nephrotoxic chemotherapy or assessment of potential live kidney donors (PLKD). British Nuclear Medicine Society (BNMS) guidelines published in 2004 recommended using a 4-sample slope intercept GFR protocol (SI-GFR). However updated BNMS guidelines published in 2018 recommend a single sample GFR measurement (SS-GFR) for patients without ascites, oedema or other expanded body space. The recommended single sample time should be based on the best available estimate of body surface area normalised GFR. One such measurement is the estimated GFR (eGFR) based on serum creatinine concentration. The purpose of this audit was to compare SI-GFR and the SS-GFR measurements to aid confidence in transitioning to a SS-GFR method. We also wished to look at the validity of using eGFR as a single sample time predictor. Materials and Methods: Using 4-sample SI-GFR measurements, the equivalent SS-GFR was calculated using the single sample time point identified in the guidelines. The patient’s most recent eGFR measurement was used to identify which of the 4 blood samples would be the SS equivalent. Results: Thirty 99mTc-DTPA SI-GFR patients were retrospectively reviewed. The thirty patients comprised 21 pre-chemotherapy assessment and 9 PLKD patients with an average eGFRs of 84.9 ± 61.8 mL/min/1.73m2 . The average SI-GFR and SS-GFRs were 84.2 ± 50.4 and 83.9 ± 53.5 mL/ min/1.73m2 respectively. Time between eGFR and mGFR was 23 ± 83 days. Comparison of eGFR measurements with SI-GFR and SS-GFR were statistically different. Recommended single sample times for eGFRs > 50 mL/min/1.73m2 demonstrated no clinical significance between SI-GFR and SS-GFR (p>0.05) however SSGFRs for patients with eGFRs of 25-50 mL/min/1.73m2 were consistently higher than SI-GFRs. Average time between eGFR and mGFR for all patients was 11.7 ± 4.4 days. Conclusion: Preliminary findings using eGFR to determine single sample times for 99mTc-DTPA mGFRs gave mixed results. As anticipated eGFR is not a good predictor of SS-GFR or SI-GFR measurements. With regard to mGFR methods, SS-GFRs at 2, 3 and 4 hours do correlate with SI-GFRs. However low GFRs estimated at 25-50 mL/min/1.73m2 (6 hour SS-GFR) give significantly different results from SI-GFRs. It may be that using the 6 hour single sample improves accuracy for lower GFRs