28th May 2025

A 29-year-old man presents with three weeks of diarrhoea and fatigue.

A 29-year-old man presents with three weeks of diarrhoea and fatigue. He is opening his bowels four times a day, and the stool has become much softer than usual.

Blood tests show:
Hb 114 g/L (130 – 180)
MCV 88 fL (80 – 100)
Ferritin 8 ng/mL (20 – 230)

He is started on iron supplements and is fully compliant; however, the fatigue is persisting despite the bloods at the next appointment showing an increase in ferritin level to the normal range.

Which blood test is likely to provide the most diagnostic yield?

Select the SINGLE most appropriate option from the list below. Select ONE option only.