An Increase in the Immature Platelet Fraction Predicts Reconstitution of the Platelet Count and May Reduce Prophylactic Platelet Transfusions after Stem Cell Transplantation.

A new automated method to reliably quantitate reticulated platelets, expressed as the immature platelet fraction (IPF), has been developed on an automated cell counter (XE-2100, Sysmex). The IPF is identified by flow cytometery using a polymethine dye, staining platelet RNA, in the reticulocyte channel; the results are available at the same time as the CBC. The IPF normal range is 1.1–6.1%, mean 3.4%, 2 SD 2.3%. Reproducibility and stability results over 48 hours were acceptable. The IPF is raised when there is increased peripheral consumption/destruction. In untreated idopathic thrombocytopenic purpura, n = 12, mean 22.3%, range 9.2–33.1% and active thrombotic thrombocytopenic purpura, n = 5, mean 17.2%, range 11.2–30.9%. Patients who may require prophylactic platelet transfusion, usually at threshold counts less than 10 x 10⁹/L, to support periods of marrow aplasia were monitored daily for platelet count and IPF%. The recovery phase of thrombocytopenia in most chemotherapy (n=13) and stem cell/bone marrow transplant patients (n=15) was preceded by a rise in IPF% several days prior to platelet recovery, mean IPF 13.7%, range 7–27.3%. In particular, patients undergoing autologous transplantation (n=8) using peripherally collected stem cells have a very characteristic IPF% motif, with a rise 1 day prior to engraftment for all patients except one where it was 2 days prior. For bone marrow derived transplant patients the increase in IPF was more variable, the rise preceded the rise in platelet count by 2–7days. These patients suffer more septic episodes where there is a rise in the IPF with no immediate increase in the platelet count, and require more regular platelet transfusions. Following a platelet transfusion there is a 24-hour transitory fall in the IPF response, which may impede platelet recovery. A parameter that could predict the timing of platelet recovery could be used clinically to reduce the use of prophylactic platelet transfusion in these patients, thus minimising donor exposure, infection risk and allowing substantial financial savings. The IPF is a useful parameter in the evaluation of the thrombocytopenic patient and has the potential to allow more optimal transfusion of platelet concentrates.