Bone marrow aspirate and biopsy with flow cytometry, cytogenetic testing, and reverse transcriptase-polymerase chain reaction (rtPCR) or fluorescence in situ hybridization (FISH) are done to identify the FIP1L1/PDGFRA - associated fusion gene or other common fusion transcripts and to evaluate for clonality of the T cell-receptor to exclude the lymphocytic variant of hypereosinophilic syndrome and other possible causes of eosinophilia.