Does this test have other names?

5-HT test

What is this test?

This test is sometimes used to help to diagnose carcinoid syndrome. This problem can happen 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. 

Why do I need this test?

You may need 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

  • Tiredness (fatigue)

People with carcinoid syndrome may also have symptoms of heart valve disease and tightening of the airways in the lungs. 

What other tests might I have along with this test?

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.

What do my test results mean?

Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you. 

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.

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

Eating foods rich in tryptophan may cause tests to come back falsely high. These include:

  • Avocados

  • Bananas

  • Eggplant

  • Hickory nuts

  • Kiwis

  • Pecans

  • Pineapple

  • Plantains

  • Plums

  • Tomatoes

  • Walnuts

Medicines, such as MAO inhibitors and lithium, can also affect the results of the serotonin test. 

How do I get ready for this test?

Avoid eating foods that can affect your test results (listed above) for at least 24 hours before the test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.