Lymphocytosis is a common laboratory finding. Based on the description of two case reports, a reactive and a malignant lymphocytosis are discussed. In addition, we describe the laboratory decision-making process in relation to a differential blood count. Automated-haematology analysers rapidly analyse whole blood samples for the complete blood cell count and leukocyte differentiation. Based on proprietary algorithms, haematology analysers will flag high or low numbers and possibly abnormal populations of leukocytes, including atypical lymphocytes. After flagging, examination of the peripheral blood smear is essential to confirm the automated leukocyte differential. Distinguishing a reactive lymphoid proliferation from a lymphoproliferative disorder requires examination of lymphocyte morphology for pleomorphic lymphocytes versus a monomorphic population, with the latter favouring a lymphoproliferative neoplasm. Samples of suspected lymphoproliferative disorders can be confirmed and further characterised by flow cytometry.
Conflict of interest: none declared. Financial support: none declared.