30th April 2024

A 45-year-old female patient presents with a four-week history of increasing fatigue and shortness of breath on exertion.

A 45-year-old female patient presents with a four-week history of increasing fatigue and shortness of breath on exertion. She is a non-smoker with a vegetarian diet. Physical examination reveals mild pallor and slight glossitis.

Investigations:

Haemoglobin: 90 g/L (120–160)

MCV: 105 fL (80–96)

White cell count: 3.8 × 10⁹/L (4.0–11.0)

Platelets: 180 × 10⁹/L (150–400)

Serum vitamin B12: 150 picomols/L (148–600)

Serum folate: 4 ng/mL (3–13)

Which additional investigation is most appropriate to establish a definitive diagnosis?