Most sensitive indicator for Iron deficiency anaemia?

Ref: CMDT 2019, P 512* Low ferritin levels <12ng/ml is a highly reliable indicator of reduced iron stores. Serum ferritin can increase in response to inflammation and hence normal or elevated ferritin level does not exclude diagnosis of IDA.* As iron deficiency progresses iron levels will be less than 30mcg/dl and transferrin levels rise to compensate.* The elevated transferrin leads to lower saturation to <15%.