quantification of 5-hydroxyindoleacetic acid in cerebrospinal fluid or urine. 5-HIAA is a metabolite of serotonin. This measurement is performed during diagnosis of serotonin secreting neuroendocrine tumours. Low levels of 5-HIAA in the cerebrospinal fluid have been associated with aggressive behavior and suicide by violent means, correlating with diminished serotonin levels. Elevated serotonin (hyperserotonemia) is one of the most common biological findings in autism[3] and 5-HIAA may be elevated in patients with autistic spectrum disorders.

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