In midgut NETs serotonin production is most prominent. Patients with NETs producing excessive serotonin can develop carcinoid syndrome. Serotonin is a product of the 'tryptophan pathway'. Serotonin is metabolized in 5-hydroxyindolacetic acid (5-HIAA) and then excreted with urine. 5-HIAA in 24-hour urine collection is one of the parameters used for follow up in patients with a serotonin-producing neuroendocrine tumor. This measurement has some disadvantages; it could be stressful, the collection is not always precisely, furthermore the collection could be problematic, for example in patients with diarrhea.