Vitamin K Status and Mortality After Kidney Transplantation: A Cohort Study

Background: Vitamin K modulates calcification by activating calcification inhibitors such as matrix Gla protein (MGP). In kidney transplant recipients, vitamin K insufficiency is common, but implications for long-term outcomes are unclear. Study Design: Single-center observational study with a longitudinal design. Setting & Participants: 518 stable kidney transplant recipients; 56% men; mean age, 51 +/- 12 (SD) years; and a median of 6 (IQR, 3-12) years after kidney transplantation. Factor: Plasma desphosphorylated-uncarboxylated MGP (dp-ucMGP) levels, reflecting vitamin K status. Outcomes: All-cause mortality and transplant failure. Results: At inclusion, median dp-ucMGP level was 1,038 (IQR, 733-1,536) pmol/L, with 473 (91%) patients having vitamin K insufficiency (defined as dp-ucMGP > 500 pmol/L). During a median follow-up of 9.8 (IQR, 8.5-10.2) years, 152 (29%) patients died and 54 (10%) developed transplant failure. Patients in the highest quartile of dp-ucMGP were at considerably higher mortality risk compared with patients in the lowest quartile (HR, 3.10; 95% CI, 1.87-5.12; P for trend <0.001; P for quartile 1 [Q1] vs Q4 <0.001). After adjustment for potential confounders, including kidney function and exclusion of patients treated with a vitamin K antagonist, this association remained significant. Patients in the highest quartile also were at higher risk of developing transplant failure (HR, 2.61; 95% CI, 1.22-5.57; P for trend = 0.004; P for Q1 vs Q4 = 0.01), but this association was lost after adjustment for baseline kidney function (HR, 1.20; 95% CI, 0.52-2.75; P for trend = 0.6; P for Q1 vs Q4 = 0.7). Limitations: Although MGP exists as various species, only dp-ucMGP was measured. No data were available for vascular calcification as an intermediate end point. Conclusions: Vitamin K insufficiency, that is, a high circulating level of dp-ucMGP, is highly prevalent in stable kidney transplant recipients and is associated independently with increased risk of mortality. Future studies should address whether vitamin K supplementation may lead to improved outcomes after kidney transplantation.