Earliest change in iron deficiency anaemia is

Ferritin is a blood cell protein that contains iron. A ferritin test helps your doctor understand how much iron your body is storing. If a ferritin test reveals that your blood ferritin level is lower than normal, it indicates your body's iron stores are low and you have iron deficiency Ref:guyton and hall textbook of medical physiology 12 edition page number:118,119,120