The objective of this observational laboratory study is to measure the correlation between 5-hydroxyindolacetic acid (5-HIAA) in platelet rich plasma and 5-HIAA in 24-hours urine collection in adult patients with a serotonin-producing neuro-endocrine tumor. Plasma and urine of included patients are collected and 5-HIAA values are measured with LCMS-MS and analysed.