Anemia occurs in up to 25% of the US population. Normal hemoglobin levels vary by race, sex, and age. Classification of anemia by mean corpuscular volume guides the differential diagnosis and evaluation. Iron studies, reticulocyte count, the red blood cell distribution width index, and blood test results are used to make the diagnosis. Iron deficiency anemia is the most common microcytic anemia and is managed with iron therapy. Parenteral iron is available when the oral route cannot be used. Patients who do not benefit from therapy should be evaluated for adherence, malabsorption, occult bleeding, systemic disease, or less common inherited disorders. A source of gastrointestinal bleeding is found in 60% to 70% of patients with iron deficiency anemia who are referred for endoscopy. Normocytic anemia has a broad differential, including nutritional deficiencies, blood loss, renal disease, malignancy (solid tumors or hematologic cancer), rheumatologic disorders, endocrine disorders, and other systemic diseases. Macrocytic anemias are seen with vitamin B12 and folate deficiency, alcohol use, thyroid disease, hydroxyurea, antiretroviral drugs, myelodysplastic syndromes, and myeloma. Oral vitamin B12 is underused, and can be as effective as intramuscular vitamin B12 in managing anemia due to vitamin B12 deficiency.
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