Anemia can be classified as acute (sudden in onset) or chronic and may range in severity from mild to moderate to severe. Moderate to severe anemia can be life-threatening because red blood cells are responsible for carrying and delivering oxygen to the rest of the body. When organs such as the brain, heart, and kidneys are deprived of oxygen, they will start to fail.

There are numerous causes of anemia. Acute anemia is most commonly due to blood loss or to destruction of the red blood cells. The source of blood loss may or may not be obvious; hemorrhage can occur due to an external wound, for example, but internal bleeding is also possible. Sources of internal bleeding include hemorrhage into the lungs, joints, GI tract, and body cavities (abdomen is most common). Alternatively, red blood cells can be destroyed secondary to immune-mediated, toxic, infectious, or cancerous processes.

Chronic anemia may be from iron-deficiency, chronic or inflammatory disease, bone marrow dysfunction, or kidney disease, among other causes. Chronic anemia may be better tolerated until it becomes severe because the body has had time to adjust to the lower red blood cell count. With severe anemia, however, the body cannot acclimate regardless of chronicity.

It is important to identify the cause of anemia and to provide blood transfusions if necessary for moderate to severe anemia.