Extramedullary Hematopoiesis in Serous Cavity Fluids: A Closer Look at This Rare Phenomenon With Diagnostic Pitfalls and Prognostic Significance.

Background: Extramedullary hematopoiesis (EMH) is usually seen in the reticuloendothelial system such as the spleen and liver; however, there have been rare case reports when EMH is seen in serous fluids (SFs). The aim of this study included analyzing the cytomorphological features of EMH in SFs in correlation with various clinicopathologic parameters and recognizing potential diagnostic pitfalls as well as their prognostic significance.

Methods: Clinicopathologic parameters and radiologic and pathologic information from the patients with a cytologic diagnosis of EMH were evaluated with cytology slides.

Results: The cytomorphologic features of EMH and diagnostic pitfalls were evaluated. Seven patients with cytologically determined EMH in SF samples, including five pleural fluids, one ascitic fluid, and one cerebrospinal fluid, were identified over the past 20 years at a comprehensive cancer center. Their mean age was 67.5 years. Most patients (n = 5) had a history of advanced myelofibrosis.

Conclusions: This study uncovered methods to differentiate EMH from peripheral blood (PB) contamination in samples in the SF. PB contamination is an important differential for EMH, and cytomorphology remains a salient parameter for the diagnosis. The comparison of the number of red blood cells and white blood cells in the PB and SF helped to distinguish EMH from PB contamination when megakaryocytes were absent. The study showed that most patients died within a year of their EMH diagnosis in SF, suggesting a strong prognostic association of this finding with poor survival outcomes.