Serum free hemoglobin is a blood test that measures the level of free hemoglobin in the liquid part of the blood (the serum). Free hemoglobin is the hemoglobin outside of the red blood cells. Most of the hemoglobin is found inside the red blood cells, not in the serum.
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
No preparation is necessary.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
Hemoglobin (Hb) is the main component of red blood cells. It is a protein that carries oxygen. This test is done to diagnose or monitor the severity of hemolytic anemia. This is a disorder in which a low red blood cell count is caused by the abnormal breakdown of red blood cells.
Plasma or serum from healthy people may contain up to 5 mg/dL hemoglobin.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
A higher-than-normal level may indicate:
- A hemolytic anemia
- Condition in which red blood cells break down when the body is exposed to certain drugs or the stress of infection (G6PD deficiency)
- Low red blood cell count due to red blood cells breaking down sooner than normal
- Blood disorder in which red blood cells are destroyed when they go from cold to warm temperatures (paroxysmal cold hemoglobinuria)
- Sickle cell anemia
- Disorder in which there is excessive breakdown of hemoglobin (thalassemia)
- Transfusion reaction
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Blood hemoglobin; Serum hemoglobin
Bunn HF. Approach to the anemias. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 161.
Elghetany MT, Banki K. Erythrocytic disorders. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 32.
Update Date 2/24/2014
Updated by: Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.