Also Known As
HIAA
Serotonin Metabolite
Formal Name
5-hydroxyindoleacetic Acid
This article was last reviewed on
This article waslast modified on
January 15, 2018.
At a Glance
Why Get Tested?

To help diagnose and monitor treatment for a carcinoid tumor that produces serotonin

When To Get Tested?

When you have symptoms suggestive of a carcinoid tumor, such as flushing, diarrhea, and/or wheezing; at intervals following treatment

Sample Required?
Test Preparation Needed?

You may be instructed to avoid certain foods and medications prior to this test.

You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

5-hydroxyindoleacetic acid (5-HIAA) is the primary metabolite of serotonin, a chemical substance (neurotransmitter) that transmits messages between nerve cells. This test measures the amount of 5-HIAA in the urine.

Serotonin is produced as needed by the nervous system, mainly the brain, but also special cells in the bronchial tubes (lungs) and gastrointestinal (GI) tract. It helps transmit nerve impulses and constrict blood vessels, participates in the wake-sleep cycle, and affects mood. After it is used by the body, serotonin is broken down in the liver, and its metabolites, including 5-HIAA, are excreted in the urine.

Ordinarily, only small varying amounts of 5-HIAA are present in the urine. Large quantities of serotonin and 5-HIAA may be produced, however, by some carcinoid tumors. Carcinoid tumors are slow-growing noncancerous or cancerous neuroendocrine masses that can form in the GI tract, especially in the appendix, and in the lungs. About 2 out of every 3 carcinoid tumors are found in the GI tract with most of the rest occurring in the lungs, although they may affect other organs as well.

According to the American Cancer Society, there are about 8,000 neuroendocrine tumors or cancers that start in the GI tract and 4,000 that start in the lungs diagnosed each year in the United States. Many more of these tumors may exist, but most remain small and do not cause any symptoms. When carcinoid tumors are discovered in asymptomatic patients during surgical procedures performed for other reasons, they are called "incidental" tumors. A small percentage of these tumors may eventually grow large enough to cause obstructions in the intestines or bronchial tubes of the lungs.

About 10% of carcinoid tumors, primarily those found in the GI tract with liver involvement, will produce enough serotonin to cause symptoms, such as flushing of the face, diarrhea, a rapid heart rate, and wheezing, which are referred to as carcinoid syndrome. The serotonin that causes carcinoid syndrome may be released continuously or intermittently and can lead to significantly increased quantities of 5-HIAA in the urine.

How is the sample collected for testing?

For a 24-hour urine collection, all of the urine produced should be saved for a 24-hour period. It is best to keep the sample in a cool, dark place, such as a refrigerator.

Is any test preparation needed to ensure the quality of the sample?

Pre-sample preparation is important for accurate 5-HIAA test results. Foods such as avocados, bananas, pineapples, plums, walnuts, tomatoes, kiwi fruit, and eggplant can interfere with 5-HIAA measurement and should be avoided for 3 days prior to and during urine collection. There are also a variety of drugs that can affect the 5-HIAA test. It is important that those being tested talk to their healthcare provider before decreasing or discontinuing any medications.

Accordion Title
Common Questions
  • How is it used?

    The 5-hydroxyindoleacetic acid (5-HIAA) urine test is used to help diagnose and monitor carcinoid tumors. It may be ordered by itself or along with a blood serotonin and/or chromogranin A level. 5-HIAA is the primary metabolite of serotonin that is excreted in the urine. Concentrations of 5-HIAA may be significantly increased when a person has a carcinoid tumor that produces serotonin.

    A 24-hour urine sample is preferred for the 5-HIAA test because the level of the metabolite can vary during the day. A random urine sample is sometimes tested, usually along with a urine creatinine level, when a 24-hour sample is not feasible. The random sample is not as accurate, however, and if excess 5-HIAA is released intermittently, then it may be missed.

  • When is it ordered?

    This test is primarily ordered when a person has symptoms suggestive of a carcinoid tumor.

    Examples of symptoms include:

    • Flushing of the face and neck (appearance of deep red color, usually with sudden onset)
    • Diarrhea, nausea, vomiting
    • Rapid heart rate
    • Wheezing, coughing, difficulty breathing

    This test may also be ordered at intervals to help monitor the effectiveness of treatment in people who have been diagnosed with and treated for a serotonin-secreting carcinoid tumor.

  • What does the test result mean?

    A significantly increased level of 5-HIAA in a 24-hour urine sample in a person with carcinoid syndrome symptoms is suggestive but not diagnostic of a carcinoid tumor. In order to diagnose the condition, the tumor itself must be located and a sample of it examined (biopsy). The health practitioner will frequently follow an abnormal test result with an order for an imaging scan to help locate any tumor(s) that may be present.

    Someone with symptoms may still have a carcinoid tumor even if the concentration of 5-HIAA is normal. The person may have a tumor that does not secrete serotonin or one that secretes it intermittently.

    A person with no symptoms and normal levels of 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumor.

    In those who are being monitored following treatment for carcinoid tumor, decreasing levels of 5-HIAA indicate a response to treatment, while increasing or continued excessive concentrations indicate that the treatment has not been successful.

  • Is there anything else I should know?

    There are a variety of drugs that can affect the 5-HIAA test. Medications that can increase 5-HIAA include acetaminophen, caffeine, ephedrine, diazepam (Valium), nicotine, glyceryl guaiacolate (an ingredient found in some cough medicines), and phenobarbital. Medications that can decrease 5-HIAA include aspirin, ethyl alcohol, imipramine, levodopa, MAO inhibitors, heparin, isoniazid, methyldopa, and tricyclic antidepressants. People should talk to their healthcare provider before decreasing or discontinuing any medications.

  • Why do I have to collect my urine for 24 hours?

    The concentration of 5-HIAA in the urine varies throughout the day. By collecting all urine for 24 hours, the amount of 5-HIAA in the urine can be averaged over the entire day and increases of 5-HIAA will be detected more easily than in a single random urine sample.

  • Are there other metabolites of serotonin?

    Yes, the other major metabolite is 5-hydroxytryptphol (5-HTOL). This substance is not routinely tested for but may sometimes be performed and the result included in a ratio with the 5-HIAA test to evaluate ethyl alcohol ingestion. An increased ratio of 5-HTOL to 5-HIAA can be indicative of alcohol consumption in both living people and post-mortem.

  • Will my results be accurate if I must continue to take my medication?

    If the drug is one that can increase or decrease the amount of serotonin and 5-HIAA, then your results may be affected. However, it is up to your healthcare provider and you to decide whether or not your medication can be safely stopped prior to and during the test collection. If your drugs must be taken, then your health practitioner will interpret the test results with this in mind.

  • Are some people at a higher risk for developing a carcinoid tumor?

    Anyone at any age can develop a carcinoid tumor but, according to the American Cancer Society, the average age at diagnosis is usually in the early 60s. People with a family history of multiple endocrine neoplasia (MEN1), a genetic condition that increases a person's risk of developing tumors in the endocrine system glands, may be at higher risk for developing a carcinoid tumor.

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