5-HIAA
Also known as: 5-hydroxyindoleacetic acid; HIAA; Serotonin metabolite
Formal name: 5-hydroxyindoleacetic acid
Related tests: Serotonin; Chromogranin A
The Test
How is it used?
5-HIAA may be ordered by itself or along with a blood serotonin level to help diagnose and monitor carcinoid tumors. A 24-hour urine sample is preferred for the 5-HIAA test because the metabolite level in the urine can vary during the day. A random urine sample is sometimes tested, usually along with a urine creatinine level, when a 24-hour sample is not feasible. The random sample is not as accurate, however, and if the excess 5-HIAA is released intermittently, then it may be missed.
When is it ordered?
This test is primarily ordered when a patient has symptoms suggestive of a carcinoid tumor. It may also be ordered at intervals to help monitor the effectiveness of treatment in patients who have been diagnosed with and treated for a serotonin-secreting carcinoid tumor.
What does the test result mean?
A significantly increased level of 5-HIAA in a 24-hour urine sample in a patient with carcinoid syndrome symptoms is suggestive but not diagnostic of a carcinoid tumor. In order to diagnose the condition, the tumor itself must be located and a sample of it examined. The doctor will frequently follow an abnormal test result with an order for an imaging scan to help locate any tumor(s) that may be present.
A patient with symptoms may still have a carcinoid tumor even if the concentration of 5-HIAA is normal. The patient may have a tumor that does not secrete serotonin or one that secretes it intermittently. A patient with no symptoms and normal levels of 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumor.
In patients who are being monitored following treatment for carcinoid tumor, decreasing levels of 5-HIAA indicate a response to treatment, while increasing or continued excessive concentrations indicate that the treatment has not been successful.
Is there anything else I should know?
There are also a variety of drugs that can affect the 5-HIAA test. Medications that can increase 5-HIAA include acetaminophen, caffeine, ephedrine, diazepam (Valium), nicotine, glyceryl guaiacolate (an ingredient found in some cough medicines), and phenobarbital. Medications that can decrease 5-HIAA include aspirin, ethyl alcohol, imipramine, levodopa, MAO inhibitors, heparin, isoniazid, methyldopa, and tricyclic antidepressants. Patients should talk to their doctor before decreasing or discontinuing any medications.
