5-HIAA

This test measures the amount of 5-hydroxyindoleacetic acid (5-HIAA) in the urine. 5-HIAA is a muscle stimulant and the primary metabolite of serotonin, a hormone derived from the amino acid tryptophan. Serotonin is produced as needed by the nervous system, mainly the brain, but also special 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 has been used by the body, serotonin is broken down in the liver, and its metabolites, including 5-HIAA, are excreted in the urine.

Normally, monly small 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 that can form in the gastrointestinal tract, on the appendix and in the lungs. According to the UK Cancer Register the incidence of carcinoid tumours in the UK is approximately 1 per 100,000 each year. Many more of these tumours may exist, but most remain small and do not cause any symptoms. When carcinoid tumours are discovered in asymptomatic patients 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, usually only after the tumour has spread to the liver. These symptoms are referred to as the carcinoid syndrome. The serotonin that causes the carcinoid syndrome may be released continuously or intermittently and can lead to significantly increased quantities of 5-HIAA in the urine.

For the 24-hour urine collection, all urine should be saved for a 24-hour period. It is better to keep sample in a cool dark place. Wrapped in a black plastic bag. When complete the sample is taken as soon as possible to your GP or the laboratory for preservation. Pre-sample preparation is important for accurate 5-HIAA test results.
For more information, see Is there anything else I should know and talk to your doctor.