If you read the literature, you will find some sources claiming that in iron deficiency anemia (IDA), the mean cell volume (MCV) falls first followed by the Hb, while other sources say the opposite (Hb drops, then the MCV).
However, in reality, either pattern may occur.
With the first pattern (MCV –> Hb), there is a window time in which the MCV has fallen while the Hb is normal. This is termed isolated microcytosis and may be mistaken for thalassemia minor.
With the second pattern (Hb –> MCV) anemia precedes a fall in the MCV. This is called normocytic iron deficiency anemia. The importance of recognizing this pattern is that we should always include IDA in our differential diagnosis of normocytic anemia.