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.

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.

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.