What is Macrocytosis (raised MCV)?

An elevated mean corpuscular volume (MCV) indicates macrocytosis, commonly defined as MCV >100 fL. It’s often picked up incidentally on routine blood tests. The clinical significance varies and should be assessed in context.

Common Causes of Raised MCV

• Alcohol excess – even without liver derangement or anaemia
• Vitamin B12 or folate deficiency – may present with anaemia or neuropsychiatric symptoms
• Liver disease – consider LFTs
• Hypothyroidism – check TSH
• Myelodysplastic syndromes – more likely in older adults with cytopenias
• Medications – e.g. hydroxyurea, zidovudine, methotrexate, phenytoin
• Reticulocytosis – e.g. post-bleed or haemolysis; reticulocytes are larger

Initial Investigations In Primary Care

• Full blood count with blood film
• B12, folate, TFTs, LFTs
• Alcohol history
• Medication review
• Reticulocyte count (if haemolysis or recovery post-bleed suspected)

When to Refer

• Unexplained macrocytosis with abnormal blood film or cytopenias
• Suspected haematological malignancy
• Persistently raised MCV despite correction of reversible causes

Red Flags

• Neurological symptoms (B12 deficiency)
• Weight loss, night sweats (possible haematological malignancy)
• Macrocytic anaemia with low WBC or platelets

Guidance References

• NICE Clinical Knowledge Summary: Anaemia – B12 and folate deficiency
• BSH Guidelines on Diagnosis of B12 and Folate Deficiency