This test is sometimes used to help to diagnose carcinoid syndrome. This problem can occur in people with carcinoid tumors. These tumors grow from a certain type of cell, and they usually show up in the lungs, stomach, small intestine, rectum, and appendix.
Some carcinoid tumors can convert a substance made from an amino acid in the body called tryptophan into a chemical called serotonin. Serotonin is further broken down to 5-HIAA. 5-HIAA is the main test for carcinoid syndrome. But a blood test to measure levels of serotonin in the blood is sometimes also used.
Relatively few carcinoid tumors—about 10% or less—are linked to carcinoid syndrome. Usually these are tumors that grow in a certain part of the small intestine and spread to the liver.
You might have this test to find out whether you have a carcinoid tumor. Symptoms of carcinoid tumor can include:
Severe skin flushing, usually in the upper body, head, and neck
Visible blood vessels in the skin, especially on the cheeks and neck
Diarrhea, possibly severe and with cramping
People with carcinoid syndrome may also have symptoms of heart valve disease and tightening of the airways in the lungs.
The main test for carcinoid syndrome measures a substance called 5-HIAA in the urine. When a tumor makes serotonin, your body turns it into 5-HIAA. This test may require collecting all the urine you make over a 24-hour period.
Another test measures a substance called chromogranin A in your blood. Higher levels may point to a larger tumor size.
A result for a lab test may be affected by many things, including the method the laboratory uses to do the test. If your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.
Higher levels of serotonin in your blood may point to carcinoid syndrome. Research found that people without carcinoid syndrome had serotonin levels in their blood of 71 to 310 nanograms per milliliter (ng/mL). In people with carcinoid syndrome, levels were 790 to 4,500 ng/mL.
But eating foods rich in tryptophan such as avocados or bananas can cause a false-positive result. For this reason, the serotonin test is not recommended for diagnosing carcinoid syndrome.
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
Eating foods rich in tryptophan may cause tests to come back falsely high. These include:
Avoid eating foods that can affect your test results (listed above) for at least 24 hours before the test. Tell your healthcare provider about any medicines you're taking.