**3.1 Retrospective studies**

As seen in Table 1, we identified four retrospective studies evaluating the relationship between dialysate sodium and interdialytic weight gain and blood pressure control. In these chart-review approaches, patients were compared in a case-control manner. In two studies, 58 patients dialyzed against the same sodium bath of 143mEq/L (Keen & Gotch, 2007; Levin et al., 2001). Patient's pre-dialysis serum sodium 'set point' was compared to the dialysate sodium resulting in a positive or negative "sodium gradient." Patients with a negative gradient had a serum sodium concentration greater than the dialysate sodium concentration; these patients had better interdialytic weight gain and improved blood pressure control than those with a positive gradient, without any change in intradialytic hypotension. Therefore, the lower the dialysate (compared to the patient's sodium) the better the IDWG and BP control.

In the two audits, patients dialyzing against a relatively lower sodium concentration had less IDWG (Davenport 2006, 2008). In the initial study, lower dialysate sodium was correlated with an improvement in BP control (defined as decrease in pre-dialysis blood pressure or number of antihypertensives prescribed). However, in the larger follow-up study, this relationship did not hold. It must be remembered that this retrospective design cannot account for the prescribing physicians reasons for the choice of dialysate sodium. It is likely that hypotension prone patients would be prescribed a higher sodium bath and less antihypertensives.


Table 1. Four retrospective studies examining the relationship of dialysate sodium prescription on interdialytic weight gain (IDWG), Blood Pressure (BP) Control, and Intradialytic hypotension. BP control is defined as improved pre-dialysis blood pressure measures and/or reduction in number of antihypertensives prescribed. n = number of patients in the study. c/w = 'compared with'. N/A = data not available. a"Set Point" was defined as mean monthly predialysis plasma sodium concentration.
