Effect of serum dialysate calcium gradient. Multivariable extended Cox regression models accounting for non-proportional hazard (where required). Data are expressed as HR with 95% CI. Model 3: fully adjusted model, country as random effect plus adjustment for age, sex, ethnicity, comorbidities (diabetes, cardiovascular disease, liver disease, and cancer), vascular access, treatment time, Kt/V, ultrafiltration volume per session, blood hemoglobin, serum albumin, calcium-containing phosphate binders, active vitamin D and analogs, and calcimimetic agents. Model 4: model 3 plus dialysate bicarbonate.