Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to 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 band 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 an 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.

The health care provider may advise you to stop taking drugs that may affect the test (see "Special Considerations").

For infants and children:

The preparation you can provide for this test depends on your child's age and previous experiences. For specific information regarding how you can prepare your child, see the following topics:

• 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)

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.

This test may be performed when a patient has skin flushing, persistent diarrhea, and valvular heart disease, the classic symptoms that may indicate carcinoid syndrome.

Serotonin is synthesized from the amino acid tryptophan by enterochromaffin cells in the stomach and bronchi (lungs). Many patients with carcinoid syndrome will have high levels of serotonin in blood and urine.

The normal range is 101-283 ng/ml (nanograms per milliliter)

Higher-than-normal levels may indicate carcinoid syndrome.

• excessive bleeding
• fainting or feeling light-headed
• 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.