Blood hemoglobin

Alternative names

Serum hemoglobin

Definition

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

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 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.

How to prepare for the test

No preparation is necessary.

For 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 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.