Synopsis
Confirm isolated macrocytosis, distinguish megaloblastic, regenerative, toxic, hepatic, endocrine and clonal patterns, and decide when observation, causal treatment or haematology referral is appropriate.
- Confirm that macrocytosis is persistent and genuine by reviewing the laboratory reference range, prior counts, film and pre-analytical flags; MCV is an average, not a diagnosis.
- Normal haemoglobin does not exclude clinically important B12 deficiency. Ask about paraesthesia, gait, vibration and joint position, cognition, visual symptoms, diet, metformin, surgery and nitrous oxide.
- Separate megaloblastic morphology, with macro-ovalocytes and hypersegmented neutrophils, from round macrocytes or target cells in alcohol and liver disease and polychromasia in regeneration.
- Use absolute reticulocytes early. A raised result redirects the assessment towards recent bleeding, haemolysis or marrow recovery rather than a primary maturation defect.
- Review alcohol and every medicine explicitly. Hydroxycarbamide and some antiretrovirals can cause expected macrocytosis, but a known medicine does not explain new cytopenias automatically.
- Check B12, folate, liver and thyroid tests and use cause-directed haemolysis studies. Mixed iron deficiency can normalise MCV or produce a high RDW and dimorphic film.
- Progressive macrocytosis, dysplasia, monocytosis or additional cytopenias warrants haematology discussion and sometimes bone marrow assessment for myelodysplastic or infiltrative disease.
- If evaluation is reassuring and the count is stable, observation is reasonable only with a named interval, symptom safety-net and responsibility for reviewing repeat results.
Key red flags
Progressive MCV, dysplastic neutrophils, abnormal platelets, monocytosis, additional cytopenias or constitutional symptoms raises a marrow neoplasm and requires specialist review.
Paraesthesia, proprioceptive loss, gait disturbance, weakness, cognition or visual change can represent urgent B12 deficiency despite preserved haemoglobin.
Investigation priorities
Confirm persistence, expose mixed cells and detect dysplasia or artefact.
Management branches
MCV is above the laboratory reference interval while haemoglobin remains within range.
- Compare previous FBCs, all lineages, RDW and sampling context, review the film and discuss cold agglutinins, delayed processing or other analyser artefact with the laboratory when appropriate.
- First-line assessment combines absolute reticulocytes, B12, folate, liver and thyroid tests with alcohol, diet, surgery, nitrous oxide, bleeding and complete medicine history.
- Escalate neurological features, haemolysis, blasts, dysplasia or additional cytopenias immediately rather than scheduling routine observation.