5-Hydroxyindole-3-acetic acid (5-HIAA) is the most abundant metabolite of serotonin (5-hydroxytrypta- mine: 5-HT). The neurotransmitter/neurohormone serotonin is synthesized from the essential amino acid tryptophan in the enterochromaffin cells of the gut and in serotonergic neurons in the central nervous system

Ned Tijdschr Klin Chem Labgeneesk 2008, vol. 33, no. 3 Introduction 5-Hydroxyindole-3-acetic acid (5-HIAA) is the most abundant metabolite of serotonin (5-hydroxytryptamine: 5-HT). The neurotransmitter/neurohormone serotonin is synthesized from the essential amino acid tryptophan in the enterochromaffin cells of the gut and in serotonergic neurons in the central nervous system (1). Peripheral serotonin is metabolized mainly in the lung and the liver through enzymatic conversion by monoamine oxidase-A (MAO-A; EC 1.4.3.4), resulting in urinary excretion of 5-HIAA. Serotonin plays an important role in carcinoid syndrome (2, 3). We previously showed that platelet serotonin is the most discriminating marker for this disease (3). However, quantification of urinary 5-HIAA proves to be the best marker for follow-up of carcinoid patients, since platelets can be saturated with serotonin. Urinary 5-HIAA is increased in Whipple disease, celiac disease and tropical sprue. Additionally, urinary 5-HIAA can be influenced by the diet serotonin content (4). Today, analytical methods have been described to measure 5-HIAA in urine, including immunoassays, gas chromatography and liquid chromatography coupled to several detection techniques (5). These methods may suffer from interferences and are time consuming, because of necessary sample clean-up. We developed an automated on-line solid-phase extraction-liquid chromatographic method with tandem mass spectrometric detection (XLC-MS/MS) for the measurement of urinary 5-HIAA (6). This method combines two previously described methods: liquid chromatographic–tandem mass spectrometry (7;8) and on-line solid-phase extraction (SPE) coupled to high performance liquid chromatography (HPLC) with fluorometric detection (9). In our laboratory XLC-MS/MS is considered to be a promising method for several applications (10).