We’ve been talking a lot about hemolytic anemias – we talked about how to figure out if your patient has a hemolytic anemia, and we talked about the DAT as a test that you would do to determine whether your patient’s hemolytic anemia falls into the immune category (warm or cold autoimmune hemolytic anemia) or the non-immune category.

Let’s look at the causes of hemolytic anemia, in general, and try to make sense out of a big list of disorders. We’ll go into depth on each disorder in separate posts.

One good way to think about the causes of hemolytic anemia is to break them down into inherited and acquired causes. The inherited causes of hemolytic anemia usually have some kind of defect in the red cell itself. Some involve  defects in the red cell membrane (like hereditary spherocytosis and hereditary elliptocytosis) such that the membrane becomes unstable; some involve enzyme deficiencies (like glucose 6 phosphate dehydrogenase deficiency, where the patient is missing an enzyme that detoxifies the red cell); and some involve defects in globin structure or synthesis (like the hemoglobinopathies, in which there are qualitative defects in hemoglobin, or the thalassemias, in which there are quantitative defects in hemoglobin).

Acquired causes of hemolytic anemia can be immune-related (we already talked about warm and cold autoimmune hemolytic anemia), infection-related (e.g., malaria or clostridium infection), drug related (tons of drugs can elicit hemolysis), or related to something outside the red cell that is ripping cells up (these are called microangiopathic hemolytic anemias, and there are tons of different “somethings” that can rip up red cells).

Usually, the inherited hemolytic anemias are chronic in nature. The patient often has a mild anemia (or no anemia) most of the time, because the bone marrow is working at its maximum to produce more red cells to replace the ones that are being destroyed. But sometimes, these patients undergo “crises,” in which something precipitates an increase in red cell destruction (parvovirus B19, for example, which loves to attack and destroy red cells) – and the already-maxed-out bone marrow can’t compensate, leaving the patient with way less red cells than normal. Acquired hemolytic anemias are usually acute in nature. Something happens to precipitate the anemia, and it’s a one-time, big hit to the red cells.

There are different ways to work up these anemias, but one thing that is done right away is the DAT. That will tell you whether your patient’s anemia falls into the immune category or not. If the DAT is positive, you have an immune process. If it’s negative, then you have a non-immune process – and we’ll talk about how to separate out all the non-immune causes of hemolysis when we discuss each disorder.

Note: the nice image of the red cell above was rendered by Andrew Mason, and can be found at: http://www.flickr.com/photos/a_mason/19191446/.