Figure 2: Diagnostic algorithm for the workup of lymphocytosis. Pleomorphic lymphocytosis favors a reactive etiology. Correlation with clinical and laboratory testing is required. Monomorphic lymphocytes are concerning for a neoplastic process, and further workup including flow cytometric immunophenotyping and appropriate ancillary testing is required. *Pertussis infection is a reactive cause of monomorphic lymphocytosis, most often seen in pediatric populations.