End-stage renal disease (ESRD) represents the final common pathway for all progressive renal disease. As of December 2014, 58,968 people in the UK were receiving renal replacement therapy (RRT), of which almost half were receiving maintenance haemodialysis (HD; UK Renal Registry Report (2014)). The Wessex Kidney Centre cares for \~1,600 patients receiving RRT, of which \>700 receive HD. The median age of our cohort is 66.7 years, similar to that of all incident UK-based patients in 2014 (64.5 years). The RRT recipient population is however growing and, increasingly, patients are more elderly with comorbidities. These individuals are unlikely to receive transplantation and will therefore depend on dialysis indefinitely. Optimising their quality of life (QoL) should be a priority.

Currently, a standard HD prescription includes a specific volume of ultrafiltration, set to achieve a clinically derived estimate of 'target weight'. Acutely, hypervolaemia (fluid overload) can be life threatening, whilst chronically it is associated with hypertension and an increased cardiovascular risk. Accordingly, target weight is typically prescribed to allow for interdialytic weight gains. The sequential reduction of target weight to achieve the lowest possible target weight (dry weight probing), thought to improve blood pressure (BP) control, left ventricular mass index and long-term outcomes, is favoured by many nephrologists. However, this results in dehydration and is associated with increased intradialytic symptoms, including intradialytic hypotension, which itself brings an increased risk of cardiac death. Local data from the Wessex Kidney Centre suggests that patients experiencing a greater drop in systolic BP pre- to post-dialysis are more likely to experience symptomatic hypotension during dialysis. Intradialytic BP variability is also detrimental to long-term outcomes. Achieving ideal hydration is therefore important for not only reducing symptomatology, but also improving BP control and cardiovascular health in these patients. Poor cardiovascular health contributes to the reduced physical function characterising this group, which along with physical (in)activity is associated with a poorer prognosis in this patient group. Furthermore, increased rates of depression and cardiovascular disease have been associated with poor physical function in this group.

There is, however, little research documenting how dehydration impacts individuals undergoing dialysis. Specifically, few studies have assessed the link between (de)hydration and frailty, although better physical function has been associated with a higher pre-dialysis BP, which may simply reflect better hydration. Experience from the Wessex Kidney Centre suggests that dehydrated dialysis patients suffer from increasing intradialytic symptomatology and a prolonged dialysis recovery time, during which they complain of prolonged fatigue. However, no studies have documented whether improving the hydration of dialysis patients improves not only their intra- and interdialytic symptomatology, but also their physiological/cognitive function, fatigue, physical (in)activity and, ultimately, QoL. Establishing the relationships between dehydration and these outcomes is essential, since hydration status can be easily and rapidly adjusted. One reason there is limited research in this area is that hydration status of individuals undergoing dialysis is difficult to objectively assess, which itself warrants further investigation.