At a glance

Also known as

5-hydroxyindoleacetic acid; HIAA; serotonin metabolite

Why get tested?

To help diagnose and monitor a serotonin-secreting carcinoid tumour

When to get tested?

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

Sample required?

A urine sample; preferably a 24 hour urine sample

Test preparation needed?

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

What is being tested?

This test measures the amount of 5-hydroxyindoleacetic acid (5-HIAA) in the urine. 5-HIAA is the primary metabolite of serotonin, a hormone made from the amino acid tryptophan. Serotonin is produced as needed by the nervous system, mainly the brain, but also cells in the bronchial tubes (lungs) and gastrointestinal 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 tumours. Carcinoid tumours are slow-growing masses found in the gastrointestinal tract, particularly the appendix and also in the lungs. The can be cancerous or noncancerous. They can remain small and not cause any symptoms. When carcinoid tumours are discovered in patients without symptoms during surgical procedures performed for other reasons, they are called ‘incidental’ tumours. A small percentage of these tumours may eventually grow large enough to cause obstructions in the intestines or bronchial tubes of the lungs.

About 10% of carcinoid tumours, primarily those found in the gastrointestinal tract, will produce enough serotonin to cause symptoms such as flushing of the face, diarrhoea, a rapid heart rate and wheezing, . These symptoms 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 in a 24-hour period should be saved. 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, but you should talk to your doctor before decreasing or discontinuing any medications.

The Test

How is it used?

5-HIAA may be ordered by itself or along with a platelet serotonin level  and/or a chromogranin A level ( both blood tests) to help diagnose and monitor carcinoid tumours. A 24-hour urine sample is preferred over a random urine sample for the 5-HIAA test because the metabolite level in the urine 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 practical. A random urine is not as accurate as an 24 hour urine sample, however, and if the excess 5-HIAA is released intermittently, then it may be missed if random sample is taken.

When is it requested?

This test is primarily ordered when a patient has symptoms suggestive of a carcinoid tumour. It may also be ordered at intervals to help monitor the effectiveness of treatment in patients who have been diagnosed with and treated for a serotonin-secreting carcinoid tumour.

What does the test result mean?

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

A patient with symptoms may still have a carcinoid tumour even if the concentration of 5-HIAA is normal. The patient may have a tumour that does not secrete serotonin or one that secretes it intermittently. A patient with no symptoms and normal levels of 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumour.

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

About Reference Intervals

Is there anything else I should know?

Some foods may give mildly elevated levels of 5-HIAA, especially bananas, chocolate, kiwi fruit, pineapples, plums, tomatoes and walnuts. These should be avoided for 2 days before and during the urine collection.

There are a variety of drugs that can affect the 5-HIAA test. Medications that can increase 5-HIAA include paracetamol (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, ethanol (alcohol), imipramine, levodopa, MAO inhibitors, heparin, isoniazid, methyldopa and tricyclic antidepressants. People should talk to their doctor before decreasing or discontinuing any medications.

Common Questions

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 in a ratio with the 5-HIAA test to evaluate ethanol ingestion. An increased ratio of 5-HTOL/5-HIAA can be indicative of alcohol consumption in both living people and post-mortem (following death).

Can I do the 5-HIAA urine test at home?

No, this test requires specialised equipment and must be performed in a laboratory. Not every laboratory will perform this assay and in some cases your sample will need to be sent to a reference laboratory.

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 doctor 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 doctor will interpret the test results with this in mind.

Are some people at a higher risk for developing a carcinoid tumour?

Anyone at any age can develop a carcinoid tumour but, according to the American Cancer Society, the average age at diagnosis is usually about 55 to 65. Patients with a family history of multiple endocrine neoplasia (MEN1), a genetic condition that increases a patient's risk of developing tumours in the endocrine system glands, may be at higher risk for developing a carcinoid tumour.

Last Review Date: November 28, 2018