Red cell exchange transfusion for babesiosis in Rhode Island.

We report four cases of clinically severe tick borne babesiosis treated with chemotherapy and adjunctive red cell exchange (RCE) at two Rhode Island hospitals from 2004 to 2007. All RCE procedures were performed using a Cobe Spectra device and were well tolerated without complications. The volume of allogeneic red cells used in the exchange was determined using the algorithm in the apheresis device with the input variables of preprocedure hematocrit, weight, height, an assumed allogeneic red cell hematocrit of 55 and a desired post procedure hematocrit of 27. The preprocedure level of parasitemia varied between 2.4% and 24% and the postprocedure level of parasitemia between 0.4 and 5.5% with an average overall percent reduction in parasitemia of 74%. Retrospectively, application of a new formula to calculate red cell mass appeared to correlate better with the percent reduction in parasitemia. Previous reports of RCE in babesiosis were reviewed. The reported reduction in parasitemia varied from 50% to >90%. Although a preprocedure level of parasitemia of 10% is sometimes used as a threshold for RCE in clinically severe babesiosis, this threshold does not have a firm empirical basis. No postprocedure desired level of parasitemia is indicated nor the mass of allogeneic red cells needed to achieve such a level. We conclude that current estimates of the dose of allogeneic red cells used in RCE are probably inaccurate, advocate a new formula to estimate this dose and suggest that a 90% reduction in parasitemia should be the minimally desired target of RCE in babesiosis.