The earliest sign of iron deficiency anaemia-

The first stage in the development of iron deficiency anaemia is the depletion of storage iron. The iron reserves are lost without compromise of iron supply for erythropoesis.Therefore the earliest sign is decreased serum ferritin level indicating poor tissue iron stores. Reference : Harsh Mohan textbook of pathology, 7th edition.Pg no.275,276 Ref Robbins 9/e pg 412