Pseudomonocytosis on a Sysmex XN haematology analyser masking the monocytopenia of hairy cell leukaemia in a South African woman.

Introduction: Hairy cell leukaemia (HCL) is a rare B-cell lymphoproliferative disorder characterised by medium-sized villous lymphocytes ('hairy cells') and monocytopenia in the peripheral blood. Automated full blood count (FBC) haematology analysers may spuriously count 'hairy cells' as monocytes, resulting in pseudomonocytosis.

Case presentation: A 72-year-old woman presented with symptomatic anaemia and massive splenomegaly to a regional hospital in North West province, South Africa, in June 2023. An FBC and differential count, performed on a Sysmex XN-series haematology analyser, revealed a monocytosis of 42.82 × 10⁹/L. However, a manual differential count, peripheral blood microscopy, and multiparameter flow cytometry confirmed a monocytopenia with numerous 'hairy cells'.

Management and outcome: The patient was referred to a tertiary hospital where bone marrow morphology and a BRAFV600E mutation confirmed a diagnosis of HCL. Unfortunately, she demised shortly after admission.

Conclusion: Here, we report a case of HCL where a Sysmex XN-series artifactually counted 'hairy cells' as monocytes, masking the characteristic monocytopenia. With the recent introduction of Sysmex XN-series FBC haematology analysers (Sysmex Corporation, Kobe, Japan) in National Health Laboratory Service laboratories across South Africa, we urge operators to be cognisant of the inherent limitations of FBC analysers in generating blood counts.

What this study adds: Even modern automated laboratory analysers with advanced technologies have inherent limitations. This case highlights the importance of a manual differential count and peripheral blood smear review in the era of automation.