Definition5-HIAA is a urine test that measures the amount of 5-hydroxyindoleacetic acid (5-HIAA) -- a break-down product of a hormone called serotonin.This test tells how much 5-HIAA the body is producing.Alternative NamesHIAA; 5-hydroxyindole acetic acid; Serotonin metaboliteHow the test is performedA 24-hour urine sample is needed.On day 1, urinate into the toilet when you get up in the morning.Afterwards, collect all urine in a special container for the next 24 hours.On day 2, urinate into the container when you get up in the morning.Cap the container. Keep it in the refrigerator or a cool place during the collection period.Label the container with your name, the date, the time of completion, and return it as instructed.For an infant, thoroughly wash the area around the urethra. Open a