Serum free hemoglobin test

Definition

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. Hemoglobin carries oxygen in the blood.

Alternative Names

Blood hemoglobin; Serum hemoglobin; Hemolytic anemia - free hemoglobin

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 how severe hemolytic anemia is. This is a disorder in which a low red blood cell count is caused by the abnormal breakdown of red blood cells.

Normal Results

Plasma or serum in someone who does not have hemolytic anemia may contain up to 5 milligrams per deciliter (mg/dL) or 0.05 grams per liter (g/L) hemoglobin.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your health care provider about the meaning of your specific test results.

What Abnormal Results Mean

A higher-than-normal level may indicate:

• Hemolytic anemia (due to any cause, including autoimmune and non-immune causes, such as thalassemia)
• Condition in which red blood cells break down when the body is exposed to certain medicines 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 disease
• Transfusion reaction

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person 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 lightheaded
• Multiple punctures to locate veins
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)

References

Marcogliese AN, Hensch L. Resources for the hematologist: interpretive comments and selected reference values for neonatal, pediatric, and adult populations. In: Hoffman R, Benz EJ, Silberstein LE, et al, eds. Hematology: Basic Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 159.

Means RT. Approach to the anemias. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 144.