When you have symptoms suggestive of a carcinoid tumour such as flushing, diarrhoea and/or wheezing
A blood sample taken from a vein in your arm
No test preparation is needed.
This test measures the amount of serotonin in the blood. Serotonin is a chemical derived from the amino acid tryptophan. It is produced as needed by the nervous system, mainly the brain, but also by special cells in the bronchial tubes (lungs) and gastrointestinal tract. In the blood, more than 90% of serotonin is found in the platelets. Serotonin helps transmit nerve impulses and constrict blood vessels, is a participant in the wake-sleep cycle, and affects mood. Serotonin is metabolised by the liver and its metabolites, primarily 5-HIAA (5-hydroxyindoleacetic acid, a muscle stimulant), are excreted in the urine.
Normally, serotonin is present in small varying quantities in the blood. Large quantities of serotonin and 5-HIAA may be produced continuously or intermittently by some carcinoid tumours. Carcinoid tumours are slow-growing masses that can form in the gastrointestinal tract, especially in the appendix, and in the lungs. They are one of several types of tumours that arise from cells in the neuroendocrine system – cells that are found in organs throughout the body and that have both nerve and endocrine aspects. The serotonin produced by carcinoid tumours may cause symptoms such as flushing of the face, diarrhoea, a rapid heart rate, and wheezing, especially when the tumour has spread to the liver. This group of symptoms is referred to as the carcinoid syndrome.
According to Cancer Research UK, carcinoid tumours are rare, with only 1,200 people diagnosed each year in the UK. Many more of these tumors may exist, but most remain small and do not cause any symptoms. When carcinoid tumours are discovered in asymptomatic patients during surgical procedures performed for other reasons, they are called "incidental" tumours. A small percentage of these tumours may eventually grow large enough to cause obstructions in the intestines or bronchial tubes of the lungs.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Serotonin may occasionally be requested following, a 24-hour urine 5-HIAA test to help diagnose carcinoid tumours. It is not generally used as a monitoring tool to evaluate the effectiveness of treatment or to detect recurrence of a carcinoid tumour. Monitoring tests may include 5-HIAA and Chromogranin A.
When is it requested?
This test is primarily requested when a patient has symptoms suggestive of a carcinoid tumour.
Some signs and symptoms include:
- Flushing of the face and neck (appearance of deep red color, usually with sudden onset)
- Diarrhoea, nausea, vomiting
- Rapid heart rate
- Wheezing, coughing, difficulty breathing
Serotonin may occasionally be requested as a follow-up test when 5-HIAA test results are normal or near normal.
What does the test result mean?
A significantly increased concentration of serotonin in a patient with carcinoid syndrome symptoms is suggestive but not diagnostic of a carcinoid tumour. In order to diagnose the condition, the tumour itself must be located and biopsied. The doctor will usually follow an abnormal test result with a request for an imaging scan to help locate any tumour(s) that may be present.
A patient with symptoms may still have a carcinoid tumour even if the concentrations of 5-HIAA and serotonin are normal. The patient may have a tumour that does not secrete serotonin or one that secretes it intermittently. A patient with no symptoms and normal or low concentrations of serotonin and 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumour.
Is there anything else I should know?
There are a variety of drugs that can affect the serotonin test result, including morphine, monoamine oxidase (MAO) inhibitors, methyldopa, and lithium. Patients should always talk to their doctor before decreasing or discontinuing any medications.
Serotonin concentrations may be slightly increased in patients with intestinal obstructions, acute myocardial infarction (heart attack), cystic fibrosis, and dumping syndrome. The serotonin test is not usually requested with these conditions.
Can I choose between the serotonin and 5-HIAA tests?
Your doctor will normally request measurement of 5-HIAA in a 24 hour urine collection in the first instance, with serotonin measurement being reserved for cases where the 5-HIAA result does not help. Urine 5-HIAA is preferred because it is more stable and, since it is collected for 24 hours, there is more chance of identifying increased 5-HIAA concentrations than excess serotonin that is only released intermittently.
Are some people at a higher risk for developing a carcinoid tumour?
Anyone at any age can develop a carcinoid tumour but the average age at diagnosis is usually quoted as 55 to 65. Patients with a family history of multiple endocrine neoplasia (MEN1), a genetic condition that increases a patient's risk of developing tumours in the endocrine system glands, may be at higher risk for developing a carcinoid tumour.