Summary

A 47-year-old woman with a 36-month history of Crohn's disease with one previous bowel resection treated with azathioprine was found to have a persistently raised mean corpuscular volume (MCV) on routine complete blood count testing. Despite vitamin B₁₂ replacement, her MCV remained elevated. A bone marrow biopsy was performed, which demonstrated myelodysplastic syndrome. Macrocytosis is a common abnormality in patients with Crohn's disease, most commonly due to vitamin B₁₂ deficiency and secondarily due to bacterial overgrowth. However, it is important to recognise that myelodysplastic syndrome is a common under-reported complication of azathioprine therapy. One-third of cases progress to acute myeloid leukaemia with poor prognosis. In patients with suspected myelodysplastic syndrome, azathioprine therapy should be immediately discontinued.