1Department of Nephrology, Barwon Health, VIC Australia

Aim: To develop a fluid allowance calculator (FAC) to individualise the recommended maximum fluid intake of haemodialysis (HD) patients

Background: Interdialytic weight gain (IDWG) is determined by fluid intake, residual renal function, and interdialytic interval. Excessive IDWG leads to fluid overload, hypertension, and increased mortality. It also necessitates a high ultrafiltration rate (UFR), in order to achieve the prescribed target weight (TW), which has been independently associated with mortality in observational studies. Despite the obvious clinical importance of fluid allowance (FA) as the only modifiable determinant of IDWG, no resources currently exist to guide its prescription in HD patients. Recommendation of a FA that limits the required UFR is needed. We propose a simple formula to calculate an individualised FA for HD patients according to their body weight, urine output and dialysis schedule.

Methods & Results:

Given: IDWG = (FA – urine output) x interdialytic interval

Given: IDWG = desired UFR x sessional duration x TW

Then: FA = [(desired UFR x sessional duration x TW) ÷ interdialytic interval] + urine output.

In this equation, UFR is expressed in ml/kg/hr and is set by individual units or nephrologists. Other parameters include sessional duration (hours), TW (kg), interdialytic interval (days), and urine output (ml).

While insensible loss has not been included, it is highly variable and difficult to measure. Furthermore, its exclusion will tend to underestimate FA, providing a safety buffer, and ensuring that the UFR remains below the pre-specified value.

Conclusions: A FAC could provide haemodialysis patients with an individualised FA that maintains a safe UFR, while avoiding the potential risk of malnutrition from an overly stringent FA. Prospective studies are needed to validate this tool.


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