A Factorial Randomized Controlled Trial of Correcting Anemia and Native Vitamin D Supplementation in Kidney Transplant Recipients

Sample size estimation:

The previous trial (the CAPRIT study) showed that 2.0 g/dL increase of hemoglobin (Hb) reduced 69% of 2-year decline in estimated glomerular filtration rate (eGFR) (Choukroun G, et al. J Am Soc Nephrol, 2012). Given that the annual eGFR decline in our patients with Hb level <10.5 g/dL was 1.66 (SD, 2.47) mL/min per 1.73 m2, the investigators hypothesized that the 2-year eGFR decline in the conservative anemia management group and the aggressive anemia correction group should be 3.32 (SD, 4.94) and 1.03 (SD, 4.94) mL/min per 1.73 m2, respectively. In order to compare the actual efficacy of the intervention with the assumptions above and to evaluate the need for an early termination of the trial, the investigators will perform one interim analysis using a Pocock type α-spending function when a total of 50-60% of the target sample size completed this study or dropped out. Assuming 20% of dropout or lost-to-follow, the planned sample size of 272 patients would yield a power of 90% for group comparison by using t-test with a type I error of 5%.

Regarding cholecalciferol supplementation, 1,000 IU/day would increase serum 25-hydroxyvitamin D level by 11.8 ng/mL in patients with BMI <30, as suggested by the previous trial (Gallagher JC, et al. Ann Intern Med, 2012). The investigators found in our prospective cohort study that the 98.2% of Japanese kidney transplant recipients had BMI <30, and that 10 ng/mL increase in 25-hydroxyvitamin D level was significantly associated with 0.75 mL/min/1.73 m2 less decrease in annual eGFR change independent of potential confounders (in submission). As with the anemia intervention arms above, the investigators will perform one interim analysis using a Pocock type α-spending function when a total of 50-60% of the target sample size completed this study or dropped out in order to compare the actual efficacy of the intervention with the assumptions above and to evaluate the need for an early termination of the trial. Therefore, the investigators expect 1.77 mL/min per 1.73 m2 in eGFR would be preserved by 1,000 IU/day of cholecalciferol supplementation for 2 years. Based on this assumption, this study size will provide a power of 70%.

Estimating kidney function:

In primary analyses, eGFR will be calculated by using the Japanese equation as in sample size calculation (Matsuo S, et al. Am J Kidney Dis, 2009). However, this formula has not yet been validated in kidney transplant recipients. Therefore, the investigators will use the creatinine-based CKD-EPI equation with Japanese coefficient (Stevens LA, et al. Nephrol Dial Transplant, 2010. Horio M, et al. Am J Kidney Dis, 2010) and an available formula if validated in Japanese kidney transplant recipients at the time of analysis.