Benefits of dialysate with greater calcium (Ca) concentration are reported in nocturnal hemodialysis (NHD) to prevent Ca depletion and subsequent hyperparathyroidism. Studies with patients dialyzing against 1.25 mmol/L Ca baths demonstrate increases in alkaline phosphatase (ALP) and parathyroid hormone (PTH) and increasing dialysate Ca subsequently corrects this problem. However, whether 1.5 or 1.75 mmol/L dialysate Ca is most appropriate for NHD is yet to be determined, and differences in the effect on mineral metabolism of daily vs. alternate daily NHD have also not been well defined. We retrospectively analyzed mineral metabolism in 48 patients, from 2 institutions (30 at Monash and 18 at Geelong), undergoing home NHD (8 hr/night, 3.5-6 nights/week) for a minimum of 6 months. Thirty-seven patients were dialyzed against 1.5 mmol/L Ca bath and 11 patients against 1.75 mmol/L. We divided patients into 4 groups, based on dialysate Ca and also on the hours per week of dialysis, <40 (1.5 mmol/L, n=29 and 1.75 mmol/L, n=8) or > or =40 (n=4 and 7). We compared predialysis and postdialysis serum markers, time-averaged over a 6-month period, and the administration of calcitriol and Ca-based phosphate binders between 1.5 and 1.75 mmol/L Ca dialysate groups. Baseline characteristics between all groups were similar, with a slightly longer, but nonsignificant, duration of NHD in both 1.75 mmol/L dialysate groups compared with 1.5 mmol/L. The mean predialysis Ca, phosphate, and Ca x P were similar between the 1.5 and 1.75 mmol/L groups, regardless of NHD hr/week. Postdialysis Ca was significantly greater, with 1.75 vs. 1.5 mmol/L in those dialyzing <40 hr/week (2.64+/-0.19 vs. 2.50+/-0.12 mmol/L, p=0.046), but postdialysis Ca x P were similar (2.25+/-0.44 vs. 2.16+/-0.29 mmol(2)/L(2), p=0.60). Parathyroid hormone was also lower with 1.75 vs. 1.5 mmol/L baths in the <40 hr/week groups (31.99+/-26.99 vs. 14.47+/-16.36 pmol/L, p=0.03), although this difference was not seen in those undertaking NHD > or =40 hr/week. Hemoglobin, ALP, and albumin were all similar between groups. There was also no difference in vitamin D requirement when using 1.75 mmol/L compared with the 1.5 mmol/L dialysate. Multivariate analysis to determine independent predictors of postdialysis serum Ca showed a statistically significant positive association with predialysis Ca, dialysate Ca, and total NHD hr/week. An elevated dialysate Ca concentration is required in NHD to prevent osteopenia but differences in serum markers of mineral metabolism between 1.5 and 1.75 mmol/L Ca dialysate in NHD in our study were few. This was similar for patients undertaking NHD <40 or > or =40 hr/week, although differences in the frequency of NHD may also be as important as dialysate Ca with regard to serum Ca levels. With concerns that prolonged higher Ca levels contribute to increased cardiovascular mortality, the optimal Ca dialysate bath is still unknown and further studies addressing bone metabolism with larger NHD numbers are required.