How to Calculate Ultrafiltration Rate in Hemodialysis
Ultrafiltration rate (UFR) is calculated by dividing the total volume of fluid to be removed (in mL) by the treatment time (in hours), and this value is commonly normalized to body weight (mL/h/kg) to assess cardiovascular stress and mortality risk. 1, 2
Basic Calculation Formula
The fundamental calculation is straightforward:
- UFR (mL/h) = Total fluid removal volume (mL) ÷ Treatment time (hours) 2
- Weight-normalized UFR (mL/h/kg) = [Total fluid removal (mL) ÷ Treatment time (hours)] ÷ Postdialysis weight (kg) 2, 3
The total fluid removal volume equals the difference between predialysis and postdialysis (target) weights, typically measured in kilograms and converted to milliliters (1 kg = 1000 mL). 2
Incorporation into Kt/V Calculations
Ultrafiltration volume is a critical component of the single-pool Kt/V formula, contributing to the overall dialysis dose calculation. 1
The KDOQI-recommended natural logarithm formula includes ultrafiltration:
spKt/V = -ln(R - GFAC × T) + (4 - 3.5 × R) + (UF/W) 1
Where:
- R = ratio of postdialysis to predialysis BUN 1
- GFAC = generation factor (ranges 0.0045-0.0175 depending on treatment frequency) 1
- T = treatment time in hours 1
- UF = ultrafiltration volume in liters 1
- W = postdialysis weight in kilograms 1
An older simplified version uses: spKt/V = -ln(R - 0.008 × t) + (4 - 3.5 × R) + (3 × 0.55 × Weight loss)/V 1
Clinical Targets and Safety Thresholds
The maximum safe ultrafiltration rate should not exceed 10 mL/kg/hour to minimize cardiovascular complications including myocardial stunning and mortality risk. 2
Weight-Specific Mortality Risk Thresholds
Recent evidence demonstrates that mortality-associated UFR thresholds are weight-dependent but not in a simple 1:1 ratio: 3
- Average UFR in mL/h = 3W + 330 (where W = postdialysis weight in kg) 3
- UFR associated with 20% higher mortality risk = 3W + 500 mL/h 3
- UFR associated with 40% higher mortality risk = 3W + 630 mL/h 3
- These thresholds are approximately 70 mL/h higher in men than women 3
Alternative Body Surface Area Scaling
Physiological hemodynamic arguments support scaling UFR to body surface area rather than body weight, particularly to avoid penalizing larger patients and inadvertently restricting intake in smaller, potentially malnourished patients. 4
Practical Application Algorithm
To determine required treatment time when UFR limits are considered:
T (hours) = V (mL) ÷ [10 × W (kg)] 2
Where:
- T = required treatment time in hours 2
- V = total fluid volume to remove in mL 2
- W = postdialysis weight in kg 2
- 10 = maximum safe UFR of 10 mL/kg/hour 2
Important Clinical Caveats
High ultrafiltration rates (>12.37 mL/h/kg) are independently associated with increased mortality risk, primarily from cardiovascular causes. 5 When patients require higher fluid removal volumes, extend treatment duration or increase treatment frequency rather than accepting excessive UFR. 1, 2, 5
Low ultrafiltration rates may signal subsequent weight loss and should prompt nutritional assessment rather than automatic dry weight reduction. 3
The plasma refill rate (maximum ~5 mL/kg/hour) represents the physiological limit at which extracellular fluid can replace contracting intravascular volume; UFR exceeding this rate inevitably causes hypovolemia. 2
Ultrafiltration contributes significantly to total urea clearance in dialysis (unlike in native kidney function), and must be included in dose calculations. 1 The ultrafiltration component can account for a substantial portion of the overall Kt/V value, particularly when fluid removal volumes are large. 1