5-HIAA (5-hydroxy indole acetic acid) is the main breakdown product of serotonin. The quantitative measurement in the urine that an individual excretes in a 24-hour period indicates how much serotonin is being made in the body during that time.
Certain foods and medicines must be avoided for a day or two before, and on the day of, the urine collection, since they can cause false test results. These are bananas, pineapple and its juice, red plums, avocado, walnuts, kiwi fruit, tomatoes, various cough medicines, muscle relaxing medicines, acetaminophen (Tylenol), caffeine, fluorouracil, iodine solutions, phenacetin, MOA inhibitors (certain antidepressant drugs), isoniazid and phenothiazine drugs (Compazine, Thorazine).
In the presence of Carcinoid Syndrome the amount of 5-HIAA is almost always distinctly increased above normal.