Correlations between serum bilirubin levels and diabetic nephropathy progression among Japanese type 2 diabetic patients: a prospective cohort study (Diabetes Distress and Care Registry at Tenri [DDCRT 5]).

OBJECTIVE To assess the correlations between serum bilirubin levels and diabetic nephropathy development and progression in type 2 diabetic patients. RESEARCH DESIGN AND METHODS Longitudinal data were obtained from 2,511 type 2 diabetic patients registered in a Japanese diabetes registry. To assess the independent correlations between serum bilirubin levels and either the development or progression of diabetic nephropathy, we used logistic regression analysis adjusted for potential confounders. RESULTS The median follow-up period was 503.4 days (range 238-777). The mean patient age, BMI, and HbA1c level was 65.2 years, 24.7 kg/m(2), and 7.5% (58.5 mmol/mol), respectively. Baseline serum bilirubin levels were significantly associated with the urinary albumin-creatinine ratio at baseline (P < 0.001) and 1 year after registration (P < 0.001). Multivariable adjusted odds ratios for progression from microalbuminuria to macroalbuminuria for the second, third, and fourth quartile of serum bilirubin levels were 0.89 (95% CI 0.49-1.58), 0.93 (0.47-1.83), and 0.33 (0.13-0.84), respectively, showing a statistically significant linear trend across categories (P = 0.032). However, this trend disappeared after adjustment for hemoglobin levels. CONCLUSIONS Serum bilirubin levels were associated with diabetic nephropathy progression in type 2 diabetic patients independent of possible confounders. Serum bilirubin levels might be the link in the correlation between hemoglobin levels and nephropathy progression.