What is the formula to calculate plasma volume to be replaced in plasmapheresis (plasma exchange)?

Plasma Volume Calculation for Plasmapheresis

The standard formula to calculate plasma volume for plasmapheresis is: Plasma Volume (liters) = Body Weight (kg) × 0.045 liters/kg, which represents approximately 1-1.5 plasma volumes per exchange session. 1

Primary Calculation Methods

Standard Weight-Based Formula

• Calculate plasma volume as 45 mL/kg of body weight (or 0.045 L/kg), which is the most widely used clinical approximation 1
• For typical adult exchanges, this translates to 2.5-2.7 liters (30-40 mL/kg) of plasma removed per session 2
• The International Atherosclerosis Society specifically recommends exchanging 1-2 plasma volumes per session, calculated as body weight (kg) × 0.045 L 1

Alternative Blood Volume-Based Calculation

• Plasma volume can also be derived from total blood volume using the formula: Plasma Volume = Blood Volume / (1 - Hematocrit) 1
• Total blood volume is estimated at 70 mL/kg in adults (up to 100 mL/kg in newborns) 1
• This method is particularly useful when hematocrit is abnormal, as it accounts for the proportion of blood that is actually plasma 3

Volume Replacement Protocols by Indication

ANCA-Associated Vasculitis

• Use 60 mL/kg volume replacement for patients with severe renal disease or diffuse pulmonary hemorrhage 1, 4
• Perform 7-10 treatments total, either daily until bleeding stops (for pulmonary hemorrhage) or every other day 1, 4

Hyperviscosity Syndrome (Multiple Myeloma, Waldenström's)

• Replace approximately two-thirds of the patient's plasma volume (typically 3-4 liters in adults) 1
• Use 5% human albumin solution or equal mixture of albumin and 0.9% normal saline as replacement fluid 1
• Repeat daily for 3-5 days until hyperviscosity is corrected 1

Lipoprotein Apheresis (Familial Hypercholesterolemia)

• Process 1-2 plasma volumes calculated as body weight (kg) × 0.045 L 1
• Alternatively, calculate blood volumes as plasma volume/(1 - hematocrit) 1
• For children <30 kg, plasma exchange (requiring smaller extracorporeal volume) is recommended over lipoprotein apheresis 1

Practical Considerations

Adjustments for Patient Characteristics

• Height, weight, and hematocrit are the three essential variables for accurate plasma volume prediction 3
• Modified Retzlaff equations incorporating these three parameters achieve correlation coefficients of 0.82 for men and 0.81 for women, even in patients with pathological hematocrit 3
• The relative error averages only -1.5% overall and 2.8% for very thin or obese patients when using height-weight-hematocrit nomograms 3

Efficiency Calculations

• To remove 90% of a pathogenic substance, 4-5 plasma exchange sessions are necessary 2
• Collection efficiency varies by device and should be calculated to avoid unnecessarily prolonged procedures 5
• Not all procedures require processing 3× total blood volume—calculate based on target substance removal 5

Replacement Fluid Selection

Standard Replacement

• 5% albumin solution is the most common replacement fluid for most indications 1, 4
• Can use combination of albumin and 0.9% normal saline 1

Fresh Frozen Plasma Indications

• Use FFP when coagulation factors need replacement (e.g., TTP, some coagulopathies) 6
• FFP carries higher risk of transfusion reactions and should be reserved for specific indications 6

Critical Pitfalls to Avoid

• Do not use fixed volumes without weight-based calculation—plasma volume varies significantly with body size 1, 2
• Do not ignore hematocrit in patients with abnormal values—standard formulas become inaccurate without this correction 3
• Do not process excessive blood volumes unnecessarily—calculate collection efficiency to optimize procedure duration 5
• Do not use plasma exchange as monotherapy—always combine with definitive treatment (immunosuppression, chemotherapy) as the procedure only provides temporary removal of pathogenic substances 1, 7