Primary bone marrow lymphoma: A hematological emergency in adults with fever of unknown origin.

Primary bone marrow lymphoma (PBML) represents non-Hodgkin lymphoma (NHL) that primarily arises in the bone marrow (BM) without lymphadenopathy. This condition has various definitions and can be masked by prolonged fever, leading to delayed diagnosis. We aimed to identify clinical features and risk indicators of PBML. We enrolled 269 adults with fever of unknown origin (FUO) who underwent a BM study for potential PBML. Thirty patients were diagnosed with PBML (26 and 4 patients in the training and validation cohort, respectively), and 20 patients (67%) showed initial manifestation of hemophagocytic lymphohistiocytosis (HLH). Among PBML patients in the training cohort, their median overall survival is short (8 days), with pneumonia being the most common direct cause of early mortality, followed by life-threatening HLH. Despite extremely poor prognoses, some B-cell PBML patients who survived 30 days after BM studies achieved long-term survival with rituximab-based treatment. To assist general practitioners in early PBML diagnosis when approaching adults with naïve FUO, we identified several risk indicators, including elevated serum alkaline-phosphate levels, lowered serum immunoglobulin-G levels, cytopenia in ≥2 lineages, and peripheral blood leukoerythroblastosis. Our recently published scoring system, which can predict hematological BM disease in FUO adults, showed excellent ability in recognizing PBML early, with high sensitivity and specificity. We conclude that PBML is a specific "clinical" phenotype of NHL; moreover, we have identified diagnostic clues for early identification of FUO adults with underlying PBML, which should be considered a hematological emergency once suspected in any adult with FUO.