Lymphocytosis

Lymphocytosis is an increased number of lymphocytes in the peripheral blood. It is defined by an absolute lymphocyte count above the upper limit of normal, which varies by age and laboratory reference ranges; in many adults, counts above about 3 x 10^9 per liter are considered elevated. Lymphocytosis is a laboratory finding rather than a single disease, and its significance depends on the clinical context and underlying cause.

Lymphocytosis can be reactive (physiologic) or clonal (neoplastic). Reactive lymphocytosis commonly follows infections, especially viral illnesses such as Epstein-Barr virus, cytomegalovirus, and pertussis; it can also occur after vaccination, in autoimmune conditions, during recovery from bone marrow suppression, or with splenectomy. Clonal lymphocytosis results from a lymphoproliferative disorder and is most often due to chronic lymphocytic leukemia (CLL) in adults, though other lymphoid malignancies can present with persistent lymphocytosis.

Evaluation typically begins with a complete blood count with differential and a peripheral smear to assess lymphocyte morphology. Flow cytometry of blood or bone marrow helps determine whether the lymphocytosis is clonal and identifies the immunophenotype (B-cell, T-cell, or NK-cell). Additional testing may include serologies for infectious agents, HIV testing, liver function tests, and imaging if there is lymphadenopathy or organomegaly.

Management depends on the underlying cause. Reactive lymphocytosis usually requires no specific treatment other than addressing the triggering illness. Clonal lymphocytosis, such as in CLL, requires hematology evaluation and may involve observation or therapy based on staging, risk factors, and symptoms. The prognosis is determined by the underlying condition rather than the lymphocytosis alone.