The 5-HIAA test measures 5-hydroxyindoleacetic acid, the primary metabolite of serotonin, in a 24-hour urine sample or serum to assess serotonin levels. It is used to diagnose and monitor serotonin-producing tumors, such as carcinoid tumors, which may cause symptoms like flushing, diarrhea, and wheezing.

Purpose: Detects carcinoid syndrome or neuroendocrine tumors, particularly midgut carcinoids. Elevated levels suggest excess serotonin production, though normal results don’t exclude non-functioning tumors.
Sample Type: 24-hour urine (collected with preservative like acetic acid) or serum. Urine collection requires avoiding serotonin-rich foods (e.g., bananas, pineapple) and certain medications (e.g., acetaminophen, MAO inhibitors) for 48–72 hours prior. Serum testing is less common but may be an alternative.
Procedure: For urine, collect all urine over 24 hours, starting with an empty bladder; for serum, a blood draw is used. Normal urine range is 2–9 mg/24h (10.4–46.8 µmol/24h), varying by lab.
Limitations: False positives can occur due to diet or drugs; false negatives may occur in early-stage or non-metastatic tumors. Exercise and renal function can also affect results.