Lymphocytosis: is it leukaemia and when to treat.

The causes of lymphocytosis range from the benign self-limiting response seen in certain infections to frank malignancy (Table I). It is usually possible to differentiate benign from malignant lymphocytosis on the basis of the clinical picture, the blood film, and a small number of widely available investigations. The further investigation of the few remaining cases has been greatly aided by recent developments in immunophenotyping, chromosome analysis and the detection of immunoglobulin and T-cell receptor gene rearrangement. The identification of monoclonality is not an absolute indication for treatment and the decision whether to undertake this in an individual patient must be based on the clinical picture as well as the nature of the lymphocyte involved.