Transient peripheral blood plasmacytosis and hematology analyzer scattergrams in acute dengue virus infection: case report with literature review.

INTRODUCTION: Circulating plasmablasts and plasma cells are characteristic but underrecognized features of acute dengue virus (DENV) infection because of their transient nature. Their presence on peripheral blood smears may erroneously suggest a plasma cell dyscrasia. METHODS: We report a case of secondary acute DENV serotype 3 infection in a 57-year-old traveler returning from the Caribbean that had morphologically suspected circulating plasma cells and thrombocytopenia, documented by peripheral blood smear and serial hematology analyzer scattergrams. Immunophenotypic confirmation was unavailable. We also performed a narrative review of published reports of plasmablast/plasma cell kinetics in DENV. RESULTS: An abnormal reactive lymphocyte scattergram with an increased very high-fluorescence cell signal prompted smear review, which revealed the abrupt appearance of reactive (activated) lymphocytes (25%) and suspected plasma cells (5%) on day 6 after symptom onset, coinciding with the platelet nadir, despite clinical improvement. Published reports have described circulating plasmablasts/plasma cells from day 2, peaking around day 6, and disappearing by day 12, mirroring thrombocytopenia. DISCUSSION: Increased very high-fluorescence cell signal on the lymphocyte scattergram should prompt peripheral blood smear review to assess suspected plasmacytosis because high-fluorescence gating, although sensitive, is not definitive. In the appropriate context, circulating plasma cells with thrombocytopenia should raise suspicion of DENV rather than a plasma cell neoplasm. These cells' kinetics may represent a potential adjunctive, phase-related laboratory clue that requires prospective validation.