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Hydroxyindoleacetic Acid (5HIAA)

Measurement of urinary 5 HIAA is useful in the diagnosis and monitoring of patients with the carcinoid syndrome, which consists of flushing, diarrhea, bronchoconstriction, wheezing, right heart valve disease, and fibrosis of endocardium, blood vessels, and skin. The syndrome is caused by carcinoid tumors, which are small slowly growing tumors of enterochromaffin cells. Generally, carcinoid tumors are indolent, but a subset is malignant. The carcinoid syndrome usually occurs following metastases to the liver with subsequent release of hormones directly into the systemic circulation. Approximately 85% of carcinoid tumors are located in the GI tract, 10% in the lung, and 5% in various other sites including thymus, ovary, kidney, prostate, skin, and breast.

Enterochromaffin cells are part of the neuroendocrine system and most of them produce serotonin. Excessive serotonin secretion is responsible for many of the symptoms associated with the carcinoid syndrome. Serotonin is primarily metabolized to 5 HIAA in the liver and kidney.5 HIAA is water soluble and excreted almost entirely in the urine. Measurement of urinary 5 HIAA is recommended for patients with suspected carcinoid syndrome. More than 90% of patients with the carcinoid syndrome have 5 HIAA levels that exceed 15 mg per 24 hours. If two 24-hour urine collections during spells fail to reveal an increased 5 HIAA level, the diagnosis of a functioning carcinoid is unlikely. If the clinical suspicion of carcinoid is high, then plasma obtained during a spell should be analyzed for serotonin.

Some foods and drugs will increase urinary 5 HIAA levels and should be avoided for 48 hours prior to the start of the collection. Drugs include cough syrups containing guaifensin, acetaminophen, Naproxen, melphalan, and fluorouracil. Foods include; avocado, bananas, eggplant, pineapple, plums, tomatoes, walnuts and hickory nuts. Aspirin, caffeine, Levodopa, and phenothiazines can cause false negative results.

Reference range is 0 - 6 mg/24 hours.

Specimen requirement is a 24-hour urine collection in a container with 25 mL of 50% acetic acid.

See also Serotonin and Carcinoid Syndrome for a more detailed discussion.

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