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Adult or child:
Blood is drawn from a vein (venipuncture), 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.

Infant or young child:
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

Adults:
No preparation is necessary.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

Infant test or procedure preparation (birth to 1 year)
Toddler test or procedure preparation (1 to 3 years)
Preschooler test or procedure preparation (3 to 6 years)
Schoolage test or procedure preparation (6 to 12 years)
Adolescent test or procedure preparation (12 to 18 years)

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. 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 an oxygen tension (pressure) of 100 mmHg (normal) in the blood vessels of the lungs, 95 to 98% of the Hb is combined with oxygen. In the peripheral tissues, where the oxygen tension is much lower, the oxygen readily separates (dissociates) from Hb.

Free plasma Hb separates (dissociates) into alpha-beta dimers (molecules). These are bound to a serum protein called haptoglobin and taken up by the liver. However, when the plasma Hb levels exceed 50 to 200 milligrams per deciliter (mg/dl), which is the capacity of haptoglobin to bind Hb, the free alpha-beta dimers pass into the urine. Plasma Hb, not bound to haptoglobin nor removed by the kidneys may be changed to a form called methemoglobin.

Ency. home > Test > H > Hemoglobin - serum

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