Blood Test: Somatomedin C (IGF-1)

What It Is

A somatomedin C test, also called an insulin-like growth factor-1 (or IGF-1) test, helps doctors evaluate whether a person is producing a normal amount of human growth hormone (hGH, or somatotropin).

HGH is produced by the pituitary gland, the pea-sized gland in the brain that helps control growth and the function of other glands. Somatomedin C is a protein produced in the liver and muscles that's known as a growth factor — its production is stimulated by hGH.

While hGH levels vary throughout the day depending on diet and activity levels, somatomedin C levels in the blood are more stable, making its measurement a fairly reliable indicator of how much hGH the pituitary gland is producing overall.

Why It's Done

The somatomedin C test is primarily ordered to check for pituitary gland disorders and abnormalities in growth hormone production. Symptoms such as short stature or excessive growth (gigantism) may warrant a somatomedin C test.

The test may also be used to assess a child's nutritional status, because malnutrition may reduce somatomedin C levels.

Preparation

Your child may be asked to stop eating and drinking for 10 to 12 hours before this test. On the day of the test, having your child wear a T-shirt or short-sleeved shirt can make things easier for your child and the technician who will be drawing the blood.

The Procedure

A health professional will clean the skin surface with antiseptic, and place an elastic band (tourniquet) around the upper arm to apply pressure and cause the vein to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.

After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.

What to Expect

Collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a few days.

Getting the Results

The blood sample will be processed in a laboratory. Because this isn't a common test, the results may not be available from the lab until a week or two after the sample is drawn.

Risks

The somatomedin C test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn, such as:

• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin causing a lump or bruise)
• pain associated with multiple punctures to locate a vein

Helping Your Child

Having a blood test is relatively painless. Still, many children are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.

Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help if your child looks away when the needle is being inserted into the skin.

If You Have Questions

If you have questions about the somatomedin C test, speak with your doctor.