An Unusual Case of Phagocytic Histiocytes on Peripheral Blood Smear of a Patient With Methicillin-Resistant Staphylococcus aureus (MRSA) Endocarditis.

Hemophagocytic lymphohistiocytosis (HLH) is a rare disorder of the immune system that may rapidly progress into life-threatening complications. There is uncontrolled activation of histiocytes and phagocytic macrophages, resulting in excess secretions of cytokines that manifest into systemic hyperinflammation affecting several organs. HLH may present as primary due to underlying familial or genetic causes or as secondary due to underlying conditions such as malignancies, infections, or autoimmune disorders. This case is of a seven-year-old girl who developed culture-proven MRSA infective endocarditis. She received sensitivity-tailored antimicrobial treatment. Initially unremarkable, but later her peripheral blood smear showed the presence of large histiocyte-like cells showing hemophagocytic activity of moderate grade (two to five histiocytes with hemophagocytosis/slide) on the feathered edge of the slide. Peripheral film findings triggered further workup for HLH. Her condition later deteriorated, requiring high-dependency unit care. This case highlights the significance of maintaining a high index of suspicion with subsequent laboratory work in cases that develop cardinal diagnostic criteria for clinical manifestations of HLH. It also emphasizes reviewing the feathered edges of blood smears to prevent overlooking important morphological findings.