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Macrocytosis is a term used to describe red blood cells that are larger than normal. When observed under the microscope on a blood smear, macrocytosis is reported as “slight” to ++++ (“four plus”). This observation helps in diagnosing and distinguishing between different types of anemia, which can be considered microcytic, normocytic or macrocytic, depending on the size of the red blood cells. A high MCH (mean corpuscular volume) can also indicate macrocytosis.

Macrocytosis can be observed in anemias caused by folate or Vitamin B12 deficiency (syndromes of malabsorption, malnutrition or pernicious anemia) or in alcoholism. Thyroid or bone marrow dysfunction or the use of certain immunosuppressive and anti-cancer drugs can also cause macrocytosis.

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