Immature Platelet Fraction
In the evaluation of thrombocytopenia, quantification of immature platelets may be used as an index of bone marrow thrombopoietic activity, analogous to the red cell reticulocyte count in the evaluation of anemia. Newly released platelets contain residual RNA and were originally termed reticulated platelets. Measurement of reticulated platelets is useful in differentiating whether thrombocytopenia is secondary to decreased production or peripheral destruction of platelets.
Hematology analyzers use flow cytometry and a fluorescent RNA binding dye to quantitate immature platelets. Today, they are called immature platelet fraction (IPF) instead of reticulated platelets. The IPF value is reported together with the other results of the routine complete blood count.
This automated IPF assay has been shown to be useful in differentiating aplastic from peripheral consumptive causes of thrombocytopenia. In one study (Br J Haematol. 2004;126:93-99) patients with immune thrombocytopenic purpura (ITP) demonstrated the highest IPF values of all the patients studied, indicating active platelet production in the setting of peripheral platelet destruction. Raised IPF values were seen in 73% of patients with ITP, and 100% of ITP patients with a platelet count below 50 th/uL. Patients with active thrombotic thrombocytopenic purpura (TTP) also had high IPF values. Patients with ITP and TTP in remission generally had low IPF values. Patients with thrombocytopenia secondary to chemotherapy had low IPF values, indicative of decreased platelet production.
In another study of thrombocytopenic patients (Am J Clin Pathol. 2006;125:282-287), patients with peripheral platelet destruction (ITP and DIC) had significantly higher IPF values than those with marrow suppression due to aplastic anemia and cancer. In this study, elevated IPF values (greater than 9%) were 89% sensitive for diagnosis of ITP, 100% sensitive for diagnosis of DIC, and 100% specific for being found in patients with peripheral platelet destruction. In summary, IPF is a useful test in the initial evaluation of a patient with thrombocytopenia. An elevated level is indicative of peripheral platelet destruction, for example ITP, TTP, or DIC, whereas a normal level is suggestive of decreased platelet production, for example drug-induced marrow suppression or aplastic anemia. In selected cases (for example typical ITP), the availability of this simple test may make performance of a bone marrow examination unnecessary.
Our own study demonstrated that IPF was a good predictor of platelet recovery following peripheral blood hematopoietic progenitor cell (HPC) transplantation. Fifty patients undergoing peripheral blood HPC transplantation (38 autologous and 12 allogeneic) were followed daily after transplantation with measurement of IPF, immature reticulocyte fraction, platelet count, and absolute neutrophil count. IPF recovery (defined as a value greater than 7%) occurred significantly earlier than recovery of all the other parameters (3.1 days earlier than platelet count, 3.8 days earlier than ANC, 0.6 days earlier than IRF). IPF recovered at least one day prior to platelet count in 79% of patients, and was followed by platelet count recovery within an average of 4.1 days (range 1-12 days, 76% within 5 days). IPF is one of the earliest predictors of hematopoietic recovery following peripheral blood HPC transplantation. In selected cases of marrow ablation, monitoring IPF may guide and possibly limit the use of prophylactic platelet transfusions, in view of anticipated imminent recovery of the platelet count.
Sample requirement is one 5ml purple-top tube. IPF can be run on the same sample as the complete blood count. Reference range is 1.1-7.1%.
