What is the calculation for number of units of factor VIII (FVIII) needed to correct factor VIII (FVIII) activity level for hemophilia A?

Updated: Jun 05, 2020
  • Author: Douglass A Drelich, MD; Chief Editor: Srikanth Nagalla, MBBS, MS, FACP  more...
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One unit of FVIII is the amount of FVIII in 1 mL of plasma (1 U/mL or 1%). The volume of distribution of FVIII is that of plasma, approximately 50 mL/kg. The difference between the desired FVIII activity level and the patient's native FVIII activity level can be calculated by simple subtraction and expressed as a fraction (eg, 100% - 5% = 95% or 0.95).

To determine the number of units of FVIII needed to correct the FVIII activity level, use the following formula:

Units FVIII = (weight in kg)(50 mL plasma/kg)(1 U FVIII/mL plasma)(desired FVIII level minus the native FVIII level)

As an example, an 80-kg individual diagnosed with hemophilia with known 1% FVIII activity level presents to the emergency department with a severe upper GI bleed. The correct dose of FVIII to administer in this case would be calculated as follows:

Units FVIII = (80 kg)(50 mL/kg)(1 U FVIII/mL)(0.99) = 3960

The next dose should be administered 12 hours after the initial dose and is one half the initial calculated dose. Minor hemorrhage requires 1-3 doses of FVIII. Major hemorrhage requires many doses and continued monitoring of FVIII activity with the goal of keeping the trough activity level at no lower than 50%. Continuous infusions of FVIII may be considered for major hemorrhage.  It is important to recall that response to factor replacement and half-life of product varies substantially across the population and that therapy should ideally be individualized for that patient.  Peak and trough values drawn after a treatment and just prior to next treatment can be very helpful in individualizing therapy.

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