Here is an example of how to calculate the dosage of von Willebrand factor (VWF) for a patient with severe von Willebrand disease (VWD). The preparation used most in the US for VWF replacement is Humate P, a fractionated human plasma product rich in VWF. The computation example may also be applied to replacement factor therapy for hemophilia A (factor VIII deficiency) or hemophilia B (factor IX deficiency). Another example appears in the VWD Part 2 module.
A patient with von Willebrand disease requires factor concentrate (Humate P) before a surgical procedure. Her weight is 132 lbs (60 kg) and her hematocrit is 30%. Compute the VWF dosage she needs to achieve an initial level of 100% (100 IU/dL). She is moderately obese, so we use 60 mL/kg as her blood volume conversion factor (70 mL/kg is normal).
First we compute her blood volume: 60 kg x 60 mL/kg = 3600 mL.
Next we compute plasma volume: 3600 mL x (100%-HCT%) = 3600 x 70% = 2520 mL total plasma volume.
We perform the VWF:RCo assay and determine she has <1% plasma VWF.
The dosage required is computed as follows: plasma volume (2520 mL) x (target level-initial level, 100 IU/dL or 1 IU/mL) = 2520 IU.
Note especially in the last calculation, though in the lab we express VWF as a percentage, or IU/dL, the actual dosage is calculated in mL. If you don’t make this final conversion, the result would be an improbable 252,000 IU. An overdose of VWF exposes the patient to thrombotic risk. Geo
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