A 60 year old male, presented with fever, cough and leukocytosis with absolute eosinophilia of more than 3 months, not responding to anti-histamines. Routine investigations to exclude causes of reactive eosinophilia were all negative. A brain MRI showed small acute infarcts and an echocardiogram showed hypokinesia with left ventricular ejection fraction of 40%. Clinically hypereosinophilic syndrome was suspected.

His Hb at the time of this presentation was 12.1 g/dL, WBC was 41,160 x 10⁹/L with absolute eosinophil count of 27,577 x 10⁹/L and platelet count was 264 x 10⁹/L. A peripheral blood smear showed markedly increased eosinophils and eosinophilic precursors with increased large, hypogranulated, hyperlobated and/or dysplastic eosinophils. A bone marrow biopsy and aspirate showed markedly hypercellular marrow with myeloid hyperplasia and increased eosinophils and eosinophilic precursors including morphologically abnormal forms. Blasts comprised 1-2% of cellularity.

Based on morphological findings, a FISH panel for eosinophilic leukemia was ordered which confirmed the presence of FIP1L1::PDGFRA fusion, which occurs due to a cryptic deletion of CHIC2 located at 4q12. This confirmed the diagnosis of myeloid/lymphoid neoplasms with PDGFRA rearrangement.

Myeloid/lymphoid neoplasms with PDGFRA rearrangement are very sensitive to tyrosine kinase inhibitors such as imatinib, therefore, it is very important to identify these neoplasms and distinguish them from other causes of eosinophilia.