What is Haptoglobin?

The haptoglobin blood test measures the level of haptoglobin in your blood.
Haptoglobin is a protein produced by the liver. It attaches to a certain type of hemoglobin in the blood. Hemoglobin is a blood cell protein that carries oxygen.

Haptoglobin is a protein produced by the liver that the body uses to clear free hemoglobin (found outside of red blood cells) from circulation. This test measures the amount of haptoglobin in the blood.

Hemoglobin is the iron-containing protein complex that transports oxygen throughout the body. It is normally found within red blood cells (RBCs) and very little is found free circulating in the blood. Haptoglobin binds to free hemoglobin in the blood. This forms a haptoglobin-hemoglobin complex that is rapidly cleared out of circulation by the liver so that it can be broken down and the iron recycled. Formation of the haptoglobin-hemoglobin complex also prevents hemoglobin from being filtered by the kidneys and passed into the urine, which can be toxic to the kidneys.

However, when an increased number of RBCs are damaged and/or break apart (hemolysis), they release their hemoglobin into the blood, increasing the amount of free hemoglobin in circulation. When large numbers of RBCs are destroyed, haptoglobin levels in the blood will temporarily decrease as the haptoglobin is used up faster than the liver can produce it. A decrease in the amount of haptoglobin may be a sign that you have a condition that is causing red blood cells to be destroyed or to break apart. When the binding capacity of haptoglobin is exceeded, the free hemoglobin level in circulation goes up and may cause tissue damage and/or organ dysfunction due to oxidative stress of free hemoglobin.

Increased RBC destruction may be due to inherited or acquired conditions. Some examples include transfusion reactions, certain drugs, and mechanical breakage, such as may be seen with some prosthetic heart valves. The destruction may be mild or severe, occurring suddenly (acute) or developing and lasting over a long period of time (chronic), and it can lead to hemolytic anemia. People with hemolytic anemia may experience symptoms such as fatigue, weakness, and shortness of breath and their skin may be pale or jaundiced.

Liver disease may also result in decreased haptoglobin levels as liver damage may inhibit both the production of haptoglobin and the clearing of the haptoglobin-free hemoglobin complexes.

How is the test used?

Haptoglobin testing is used primarily to help detect and evaluate hemolytic anemia and to distinguish it from anemia due to other causes. Testing is used to help determine whether red blood cells (RBCs) are breaking apart or being destroyed prematurely. It may be used in conjunction with other laboratory tests such as a complete blood count (CBC, including RBC count, hemoglobin, hematocrit), reticulocyte count, lactate dehydrogenase, bilirubin, direct antiglobulin test, and blood smear.

Though the haptoglobin test is a sensitive test for hemolytic anemia, it cannot be used to diagnose the cause of the condition. Other laboratory tests may be needed to help determine the cause, such as tests for autoantibodies to detect autoimmune causes, sickle cell tests, G6PD, or a hemoglobin evaluation.

If you have had a blood transfusion, a haptoglobin test may be ordered along with a direct antiglobulin test to help determine if you have had a transfusion reaction. (For more details, see the article on Transfusion Medicine.)

If your haptoglobin level is low, then testing may be repeated at a later time to monitor your condition, evaluate changes in concentration, and help determine whether the increased destruction of RBCs persists.