A 63-year-old Irish lawyer with healthy appearance (bronzed skin) and a normal body mass index presents to his family physician with complaints of increasing fatigue and pain in his metacarpophalangeal joints (MCP II and III) of the hands which impair him taking notes during negotiations. Screening blood tests reveal the following abnormal results: glucose 1.7 x upper limit of normal (ULN; fasted); ALT 1.6 x ULN; AST 1.5 x ULN; GGT 1.9 x ULN; ferritin 7.2 x ULN.

Which test is recommended as the next step to a possible diagnosis?