Health Encyclopedia

Serum hemoglobin

Definition

Serum hemoglobin is a test that measures the level of free hemoglobin in the blood. Free hemoglobin is hemoglobin outside the red blood cells.

Alternative Names

Blood hemoglobin; Serum hemoglobin

How the test is performed

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is first cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein.

The pressure causes veins below the tourniquet to fill with blood. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to bring back the blood flow to the arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For infants or young children:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

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, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

The test is done to detect and monitor the severity of various kinds of hemolytic anemia, or breakdown of red blood cells.

Hemoglobin (Hb), the main component of red blood cells, is a protein that carries oxygen away from the lungs and carbon dioxide back to the lungs. in a normal lung, up to 98% of the Hb is combined with oxygen. In the tissues further from the lungs, where the oxygen tension is much lower, the oxygen readily separates from Hb.

Free plasma Hb separates into two parts, called alpha and beta molecules. These are bound to a serum protein called haptoglobin and taken up by the liver. However, when the plasma Hb levels exceed 50 - 200 mg/dl, which is the capacity of haptoglobin to bind Hb, the free alpha-beta molecules pass into the urine.

Plasma Hb that is not bound to haptoglobin nor removed by the kidneys may be changed to a form called methemoglobin. Methemoglobin does not carry oxygen.

Normal Values

11.5 to 15.5 mg/dL (can vary by age, gender, and testing laboratory)

Note: mg/dL = milligrams per deciliter

What abnormal results mean

Elevated levels may indicate:

Drug-induced immune hemolytic anemia
G6PD deficiency
Hemoglobin C disease
Hereditary spherocytosis
Idiopathic autoimmune hemolytic anemia
Paroxysmal cold hemoglobinuria (PCH)
Paroxysmal nocturnal hemoglobinuria (PNH)
Sickle cell anemia
Thalassemia
Transfusion reaction

What the risks are

Excessive bleeding
Fainting or feeling lightheaded
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Multiple punctures to locate veins

Special considerations

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.

Review Date:10/30/2006
Reviewed By:William Matsui, MD, Assistant Professor of Oncology, Division of Hematologic Malignancies, The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, MD. Review provided by VeriMed Healthcare Network.

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