This is a blood test to measure the level of serotonin in the blood. It is also known as 5 – Hydroxytryptamine (5-HT) test. Serotonin is a neurotransmitter in the brain responsible for transmitting messages between nerve cells and also plays a role in the wake-sleep cycle. Normally, serotonin is present in small amounts in the blood. Large quantities may be secreted continuously or intermittently by some carcinoid tumours. The serotonin test is used as a diagnostic test for carcinoid tumours. Carcinoid tumours are tumour masses that form in the gastrointestinal tracts and the lungs. The test may be performed in individuals with symptoms like flushing of face and neck, abdominal pain, nocturnal diarrhoea, vomiting and nausea.