Laboratory workup of Hypereosinophilia

Abstract Hypereosinophilia (HE) can be caused by a wide variety of non-hematologic (secondary or reactive) and hematologic (primary, clonal) disorders. Diagnosing hypereosinophilia/hypereosinophilic syndrome (HE/HES) is challenging due to the complex nature of disease manifestations and numerous underlying etiologies. Knowing that only rare cases are clonal, it is wise to rule out reactive conditions and proceed with molecular and other advanced tools. The exclusion of secondary causes needs a detailed clinical evaluation followed by a wide range of serological and imaging investigations. Once reactive eosinophilia has been ruled out, the diagnosis of primary HE/HES is made using a combination of morphologic examination of the blood and bone marrow, conventional cytogenetics, fluorescent in situ hybridization, flow-cytometry, and T-cell clonality evaluation to look for histopathologic or clonal evidence of an underlying hematological disorder. The accurate diagnosis of clonal eosinophilia-causing myeloid and lymphoid neoplasms and the identification of numerous gene rearrangements significantly enhance patient outcomes, because a proportion of these patients (such as PDGFRA and PDGFRB rearrangements) responds well to tyrosine kinase inhibitors. Considering the complex etiopathologies, the cost of testing, and the time involved, the workup needs to be tailored according to the urgency of the situation and the resources available. In urgent situations with organ damage, it is crucial to initiate appropriate management without waiting for the results of investigations. In contrast, in a resource-limited situation, it is acceptable to employ step-by-step rather than comprehensive testing to rule out the most common causes first. Here, we discuss various laboratory investigations employed in diagnosing HE/HES, highlighting their importance in different situations.