Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is 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.

This causes veins below the tourniquet to distend (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 restore circulation. 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.

No preparation is necessary.

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.

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. A molecule of hemoglobin consists of 2 pairs of peptide (alpha and beta globins) chains and four heme groups, each with one atom of iron. At a normal oxygen tension of 100 mmHg in the blood vessels of the lungs, 95 - 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 alpha-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.

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

Note: mg/dl = milligrams per deciliter

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

• 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

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.