Pure red cell aplasia in a patient with rheumatoid arthritis after JAK inhibitor exposure-diagnostic challenges

We describe a patient with long-standing rheumatoid arthritis (RA) who developed severe refractory transfusion-dependent anaemia lasting several months following treatment with a JAK1 inhibitor (Filgotinib). The patient proceeded with extensive investigations, including bone marrow assessment, and was initially diagnosed with Filgotinib-associated anaemia. However, despite stopping this agent, the anaemia persisted with fluctuating reticulocyte count resulting in multiple hospital admissions. Although the anaemia was initially attributed to Filgotinib, its persistence alongside delayed reticulocytopenia, ultimately led to a diagnosis of acquired Pure Red Cell Aplasia more than 12 months later. Complete remission was achieved with prednisolone + ciclosporin A. This case underscores the need to consider PRCA in RA patients on JAK inhibitors even with initially normal reticulocyte counts, as these agents may transiently mask underlying aplasia.