A 30-year-old man with Crohn's disease, maintained on azathioprine, presents with increasing fatigue and pallor over several weeks. Blood tests show: Hb 95 g/L, MCV 108 fL, WBC 3.2 x 10^9/L, Platelets 110 x 10^9/L. What is the most important initial investigation to guide management regarding his azathioprine therapy?