The 5-HIAA test (5-Hydroxyindoleacetic acid) is a specialized diagnostic tool used primarily to assess the body’s serotonin production and metabolism. This test measures 5-HIAA, a breakdown product of the neurotransmitter serotonin, excreted in the urine. Serotonin is largely produced by specialized cells within the gastrointestinal tract. The primary purpose of the test is to screen for and monitor neuroendocrine tumors, specifically carcinoid tumors, which can secrete excessive amounts of serotonin, leading to carcinoid syndrome. Accurate results depend heavily on strict preparation and processing protocols.
Essential Preparation Before Testing
The most significant factor influencing the accuracy of the 5-HIAA result is the patient’s adherence to preparation protocols in the days leading up to and during the collection. Since 5-HIAA is a metabolite of serotonin, consuming foods naturally rich in serotonin or its precursors can artificially inflate the test results, leading to a false elevation. To prevent this interference, patients are instructed to follow a strict dietary restriction for at least 72 hours before the start of the collection period and throughout the full 24 hours of collection.
A detailed list of foods to avoid includes bananas, pineapples, plums, avocados, kiwifruit, and various nuts like walnuts and pecans. Certain vegetables such as eggplant and tomatoes, including tomato-based sauces and products, must also be temporarily eliminated from the diet. The rationale behind these restrictions is that external serotonin or its precursor molecules from these foods are metabolized and excreted as 5-HIAA, making it impossible to distinguish between the body’s natural production and the dietary intake.
Medications also represent a source of potential interference, and patients must consult with their healthcare provider about pausing certain prescription or over-the-counter drugs. Medications can affect the metabolism of serotonin or alter 5-HIAA levels, sometimes causing false positives or false negatives. Drugs that may increase 5-HIAA levels include acetaminophen, caffeine, cough suppressants containing guaifenesin, and nicotine, requiring their temporary avoidance.
Conversely, certain medications can decrease 5-HIAA levels, potentially masking a true elevation. These include monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants, and methyldopa. Patients must not discontinue any prescribed medication without explicit instruction and supervision from the ordering physician. This careful management of diet and pharmaceuticals over the three days preceding the test is paramount to ensuring the sample accurately reflects the body’s endogenous serotonin metabolism.
The Lab Processing Timeline
The actual timeline for receiving 5-HIAA results begins with the patient’s precise execution of the 24-hour urine collection. This process requires the patient to collect all urine voided over a continuous 24-hour period into a special container provided by the laboratory. This container often contains a chemical preservative, such as hydrochloric acid, to stabilize the 5-HIAA. The collection starts after the first morning void is discarded and concludes exactly 24 hours later with the final void.
The container must be kept refrigerated or on ice throughout the entire collection period to maintain the sample’s integrity and slow down the degradation of the analyte. Prompt submission of the sample to the laboratory immediately after the collection is complete is required to minimize any loss of the 5-HIAA compound. Once the sample reaches the clinical laboratory, the internal processing time can vary based on the facility’s capabilities.
If the testing is performed at a large, local hospital laboratory with dedicated equipment, the analytical phase usually takes between two to four business days. The laboratory will use a highly specific technique, often Liquid Chromatography–Tandem Mass Spectrometry (LC-MS/MS), to accurately quantify the 5-HIAA concentration.
However, because this is a specialized test, many smaller facilities must send the sample to a regional or centralized reference laboratory. When samples must be shipped, the total turnaround time for the result to be analyzed and reported back to the ordering physician generally extends to between four and eight business days. In some cases involving complex assays or high-volume reference labs, the total wait can be as long as seven to ten working days, or occasionally up to two weeks. This total timeline includes the patient’s 24-hour collection, transport time, the laboratory’s analytical time, and the time required for the result to be electronically transmitted to the doctor’s office.
Interpreting Your 5-HIAA Results
Once the laboratory processing is complete, the final result is a numerical value representing the total amount of 5-HIAA excreted during the 24-hour period, typically measured in milligrams per 24 hours (mg/24h). The normal reference range for adults is generally considered to be between 2 and 9 mg/24h, though this range may show slight variations depending on the specific method and equipment used by the analyzing laboratory. The ordering physician is the only person qualified to interpret the result in the context of the patient’s symptoms and preparation adherence.
A result that falls within the normal range suggests that the patient is unlikely to have a serotonin-producing neuroendocrine tumor. However, some tumors do not secrete serotonin consistently, meaning a normal result does not completely rule out the presence of a tumor. A significantly elevated 5-HIAA level, particularly one greater than 25 mg/24h, provides a strong indication of a serotonin-producing tumor, especially when the patient is experiencing symptoms of carcinoid syndrome like flushing or chronic diarrhea.
An elevated result is not a definitive cancer diagnosis but rather a biochemical signal that requires further investigation. The next steps often involve the physician ordering additional blood tests, such as Chromogranin A, which is another marker for neuroendocrine tumors. Imaging studies like CT scans, MRIs, or specialized nuclear medicine scans are also typically used to locate the tumor and determine its size and spread. The interpretation of a high 5-HIAA level is ultimately a prompt for a comprehensive diagnostic workup, guiding the physician toward confirmation and treatment planning.