2 Blood volume (ml) x (1 -hematocrit) = plasma volume (ml)
3 Plasma volume (ml) x (desired Factor VIII level u/ml - initial
Factor VIII u/mi) = units Factor VIII required
Example: To raise the Factor Viii level to 50 percent in a 70-kg patient
with a hematocrit of 40 percent and a factor VIII level of 0 percent.
70-kg x 70 mI/kg = 4,900 ml
4,900 ml x (1-0.40) = 2,940
2,940 ml x (0.5-0) = 1,470 units
(d) In cases where the initial Factor VIII level is not known, it can be
assumed to be 0 percent in a patient with severe classic hemophilia A. The half-Iife of
Factor VIII after transfusion is 8 to 12 hours, so that it is usually necessary to repeat the
Factor VIII transfusion at 8- to 12-hour intervals in order to maintain hemostatic levels.
Some hemophiliacs may have an inhibitor that causes a shortened half-life of Factor VIII
after transfusion. In calculating the dose of Factor VIII; it may be necessary to account
for other variables, such as differences in the intravascular half-Iife of transfused Factor
VIII and the extravascular distribution of Factor VIII. The calculations, described above,
provide an estimate of the Factor VIII level attained immediately after transfusion.
Dosage should be adjusted so that the minimum desired level is reached just prior to
the next infusion.
Example: If the desired minimum Factor VIII level = 30 percent and since the
half-life of Factor VIII = 12 hours, it is necessary to elevate the patient's initial
Factor VIII to 60 percent so that the level just before the next dose will be 30
percent.
Patient's plasma volume = 2,940 ml
2,940 ml x (60-0) or 1,764 units required to elevate Factor VIII to 60
percent
12 hours later, half of the Factor VIII remains, thus: the Factor VIII
level is 30 percent
For the next dose of Factor VIII, 2,940 x (0.60-0.30) or only 882 units
are required to elevate Factor VIII to 60 percent
MD0846
3-10
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