Calculating the Blood Transfusion Requirement for a Patient with Chronic, Transfusion-Dependent Anemia

transfusion requirement as grams hemoglobin per month =

= ((((initial pretransfusion hemoglobin in g/L) * (blood volume in liters))+ ((number of blood units transfused) * 75)) - ((pretransfusion hemoglobin in g/L prior to next transfusion) * (blood volume in liters))) / (number of days between transfusions) * 31

The blood volume was calculated using equations from Ganong:

plasma volume in liters =

= (body weight in kilograms) * 0.045

blood volume in liters =

= (plasma volume in liters) * 100 / (100 - (hematocrit))

where:

• 75 indicates the mean hemoglobin in 1 unit of packed RBCs as g per unit. A normal person should have a hemoglobin of about 140-150 g per liter. 1 unit of packed RBCs is made from 500 mL of blood, or 70-75 g.

• The equation from 1992 used 8.7 grams hemoglobin per unit rather than the 75 used in the later form.

• 31 indicates the number of days in a month.

• As discussed in Chapter 2 (02.10.05) the whole body hematocrit should be used. This equals 0.91 * (venous hematocrit).

• Blood volume could probably be calculated before each transfusion but I will treat it as constant just to keep the calculations simple.