Model: simple one-compartment model
Blood used for exchange transfusions:
(1) whole blood < 24 hours old (preferred)
(2) transfusion of packed cells together with fresh frozen plasma in 1:1 ratio (approximates whole blood; these should not be mixed before exchange but each product hung separately)
where:
• The default hemoglobin for whole blood was 14.4 g/dL
• The default hemoglobin for packed red blood cells was 24 g/dL.
hemoglobin following exchange transfusion in g/dL =
= (hemoglobin of transfused blood in g/dL) – (((hemoglobin of transfused blood) – (hemoglobin of patient before exchange)) * EXP((-1) * (decimal fraction of blood volume exchanged)))
percent parasitemia after parasitemia =
= (percent parasitemia before exchange) * (hemoglobin before transfusion) / (hemoglobin following exchange) * EXP((-1) * (decimal fraction of blood volume exchanged))
where:
• Percent parasitemia refers to percent of red blood cells containing parasites.
By rearranging this:
decimal fraction of blood volume exchanged =
= (-1) * LN((percent parasitemia after exchange) / (percent parasitemia before) * (hemoglobin following exchange) / (hemoglobin before exchange)) =
= LN((percent parasitemia before) / (percent parasitemia after) * (hemoglobin before exchange) / (hemoglobin after exchange))
An exchange of 0.69 blood volume will reduce the percent parasitemia to about 50% of the original, depending on the hemoglobin of the transfused blood:
fraction of original parasitemia after an exchange of 0.69 blood volume =
= (hemoglobin before exchange) / ((hemoglobin of transfused blood) + (hemoglobin before transfusion))
In a series of 29 patients, the formula tended to overestimate the final parasitemia. One possible explanation was that patients were receiving concurrent quinine and other antimalarials concurrently with the exchange transfusion.
where:
• One other explanation might be that severe falciparum malaria may show 2 ring forms per RBC.
