BAL Flow Cytometry Unmasks Nodal T Follicular Helper Cell Lymphoma-Associated Hemophagocytic Lymphohistiocytosis When Bone Marrow Is Nondiagnostic: A Case Report.

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome that often presents with nonspecific pulmonary manifestations, frequently mimicking severe pneumonia or acute respiratory distress syndrome (ARDS). We report a case of malignancy-associated HLH secondary to nodal T follicular helper (TFH) cell lymphoma presenting as nonresolving pneumonia with progressive hypoxemic respiratory failure. Bronchoalveolar lavage (BAL) flow cytometry demonstrated an aberrant CD4-predominant T-cell population (CD4:CD8 ratio 9.2:1) with loss of pan-T cell antigens CD5 and CD7, providing early evidence of pulmonary lymphoma involvement and prompting expedited hematological evaluation. Critically, bone marrow flow cytometry was nondiagnostic despite histological involvement, highlighting BAL as the higher yield diagnostic site. This case argues for routine BAL immunophenotyping in critically ill patients with nonresolving pulmonary infiltrates, hyperferritinemia, and cytopenias.