Which of the following is the most sensitive and specific test for diagnosis of iron deficiency?

The serum ferritin level correlates with total body iron stores so it is the most sensitive and specific laboratory test to estimate iron stores in the body. An average normal level of ferritin in an adult male is 100 g/L, while in adult females it is 30 g/L. As the iron stores are depleted, the serum ferritin falls to diagnostic of absent body iron stores. Ref: Harrison's Internal Medicine, 18th Edition, Chapter 103