Immunophenotyping for the Assessment of Asymptomatic Lymphocytosis: A Retrospective Analysis and National Survey.

Asymptomatic lymphocytosis poses a common challenge in haematology. Immunophenotyping can establish whether a clonal population is present, but it is expensive and the benefit of diagnosing asymptomatic patients is unproven. This study aimed to establish data to guide the use of immunophenotyping. We analysed the proportion of lymphocytosis in full blood count (FBC) samples across a five-year period within a large UK National Health Service (NHS) trust. Persistent lymphocytosis was present in 0.18% (437/242678) of repeat community samples. Of samples sent for immunophenotyping, 743/784 (95%) with a lymphocyte count > 10 × 10⁹/L had a clonal population, compared to 223/1696 (14%) with a lymphocyte count < 5 × 10⁹/L. We followed up a longitudinal cohort of asymptomatic patients with clonal lymphocytosis to determine how many required treatment. The minority (11/46) of patients needed treatment within 9 years of follow-up. Of patients needing treatment, 10/11 (91%) had a presenting lymphocyte count > 10 × 10⁹/L. In all cases, treatment was initiated when the patient became symptomatic. We propose a lymphocyte count threshold of > 10 × 10⁹/L for referral and immunophenotyping in patients with asymptomatic lymphocytosis. This approach aims to balance safety and cost-effectiveness and reflects uncertainty in the value of diagnosis for asymptomatic patients.