Introduction: Post kidney transplant (KTx) with functioning renal allograft, serum creatinine continues to decrease at a variable rate until it stabilizes. Currently, the Cockcroft- Gault (C-G) and Modification of Diet in Renal Disease MDRD-4 formulae, and are based on a single serum creatinine value to estimate GFR for chronic kidney disease. For rapid changes in serum creatinine, the kinetic estimated GFR (KeGFR)1 formula was developed for recovering acute kidney injury. However, post-KTx with functioning allograft eGFR and KeGFR formulae/calculators’ values are not compared with measured urinary creatinine clearance by any glomerular isotope excretion methods. Hence, the medications requiring dose adjustment based on renal function might not be adjusted appropriately. The aim of the present study is to compare actual urinary creatinine clearance every 12 hours with C-G, MDRD-4, and (KeGFR) after functional KTx.

Patient and method: 34 consenting subjects (mean age 55.2 +/- 13.8 years; 14 male) were enrolled of which 24 had immediate primary allograft function. These were prospectively studied under IRB-approved protocol. Urine was collected every 12 hours through a Foley catheter post-KTx until serum creatinine was stabilized. Creatinine clearance was calculated {U (urinary creatinine) x V (urinary volume)/ S (serum creatinine); (UV/S)}. Over 200 creatinine clearance values were compared with C-G, MDRD-4, and Chen’s KeGFR calculator at 12, 24- and 48-hours intervals.

Results: At each time point, the measured urinary creatinine clearance was consistently higher compared with eGFR by C-G, MDRD-4 formula, and KeGFR by Chen calculator1 until serum creatinine was stabilized around 2mg/dL (Figure-1). The mean creatinine clearance was 68.1 ± 22.1 mL/min. Twenty-two (91%) subjects had a >50% decrease in serum creatinine at the time of discharge. Of these 22 subjects, 20 (90%) had a urinary creatinine clearance >50 mL/min and 18 (81%) had a creatinine clearance of >60 mL/min.

Conclusion: Our prospective observations suggest that eGFR by C-G, MDRD-4, and KeGFR at 12, 24, and 48 hours, underestimates the GFR compared to urinary creatinine clearance for post-KTx functioning allograft recipients. We suggest adjusting post-KTx medications based on a 50% decrease in pre-transplant serum creatinine, where measured urinary creatine clearance is >50 mL/min in 92% of cases and >60 mL/min in 81% of cases. Renally dosed medication could be adjusted with a 50% decrease in serum creatinine in post-KTx. Future studies are required with exogenous glomerular filtration markers for more accurate formulae with functioning allografts in the immediate post-KTx period.

Reference:

1. https://www.mdcalc.com/kinetic-estimated-glomerular-filtration-rate-kegfr