Earliest response to iron in iron deficiency anemia is denoted by
Response to iron therapy:

Rapid subjective improvement, with disappearance (or) marked dimination of fatigue, lassitude, and other nonspecific symptoms before any improvement in anemia is observed.
Earliest heamotologic evidence of response to treatement is an increase in reticulocytes % G and Hb content.
Maximal reticulocytes % is on 5th to 10th day after institution of therapy and thereafter returns to normal.
Maximum value usually ranges from 5 to 10 % and is inversly related to the level of heamoglobin.
Heamoglobin level is the most accurate measure of the degree of anemia in iron deficiency.
Restoration of enzymes
Reticulocytosis
Increase in iron birding capacity
Increase in heamoglobin