Which of the following is the most sensitive and specific initial laboratory test to diagnose iron deficiency?

In iron deficiency anemia, the earliest parameter that is effected is - decrease in stored iron - measured by serum ferritin levels. Hence, estimation of Serum ferritin levels is most sensitive and specific test to diagnose iron deficiency anemia in the given options. However, do remember the ratio discussed in the video for an advanced question. Iron studies in normal people (A) and those with iron deficiency (B) ALSO KNOW: anemia of chronic disease (C), and iron overload diseases (D).