Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.
Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.
Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.
No special preparation is necessary for this test.
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.
You may have this test if your health care provider suspects that you have a disorder caused by abnormal forms of hemoglobin (hemoglobinopathy).
Many different types of hemoglobin (Hb) exist. The most common ones are HbA, HbA2, HbF, HbS, HbC, Hb H, and Hb M. Healthy adults only have significant levels of HbA and HbA2.
Some people may also have small amounts of HbF (which is the main type of hemoglobin in an unborn baby's body). Certain diseases are associated with high HbF levels (when HbF is more than 2% of the total hemoglobin).
HbS is an abnormal form of hemoglobin associated with sickle cell anemia. In people with this condition, the red blood cells sometimes have a crescent or sickle shape. The cells easily break down, or can block small blood vessels.
HbC is an abnormal form of hemoglobin associated with hemolytic anemia. The symptoms are much milder than they are in sickle cell anemia.
Other, less common, abnormal Hb molecules cause anemias.
In adults, these hemoglobin molecules make up the following percentages of total hemoglobin:
- Hb A: 95% to 98%
- Hb A2: 2% to 3%
- Hb F: 0.8% to 2%
- Hb S: 0%
- Hb C: 0%
In infants and children, these hemoglobin molecules make up the following percentages of total hemoglobin:
- Hb F (newborn): 50% to 80%
- Hb F (6 months): 8%
- Hb F (over 6 months): 1% to 2%
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The presence of significant levels of abnormal hemoglobins may indicate:
- Hemoglobin C disease
- Rare hemoglobinopathy
- Sickle cell anemia
There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood 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 light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
You may have false normal or abnormal results if you've had a blood transfusion within the previous 12 weeks.
Nagel R. Methemoglobinemias and unstable hemoglobins. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 168.
Steinberg M. Sickle cell disease and associated hemoglobinopathies. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 167.