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.

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.

Since October 2013 it is possible, because of a better measurement sensitivity, to measure reliably 5-HIAA in platelet rich plasma (PRP) by isotope-dilution massa-spectrometer (LCMS-MS). Recent publications show that 5-HIAA measured in PRP could have an additive value in diagnosis and follow up of patients with serotonin-producing NET.

Objective:

The primary objective is to measure the correlation between 5-HIAA in PRP and 5-HIAA in 24-hours urine collection in adult patients with a serotonin-producing neuro-endocrine tumor.

Secondary objective is to determine if increase or decrease of 5-HIAA in plasma is associated with disease progression.

Study design:

This is an observational laboratory study. Plasma and urine of included patients and collected in standard health care, are selected and the 5-HIAA's are measured with LCMS-MS. Also age, sex, diagnosis and clinical course of the patients will be collected from the medical record.

Study population:

In this study the investigators will use blood and urine of 60 patients with serotonin-producing neuro-endocrine tumors.

Main study parameters/endpoints:

The main endpoint of this study is the correlation between 5-HIAA in PRP and 5-HIAA in 24-hour urine collection. Secondary endpoint is the correlation between 5-HIAA in PRP and course of the disease, determined by CT-scan (if applicable).

Inclusion Criteria:

In order to be eligible to participate in this study, a subject must meet all of the following criteria:

• Serotonin in platelets > 5.4 nmol / 10^9 and 5-HIAA in 24-hours urine >3.8 mmol/mol creatinine
• Adult NET patients (aged ≥ 18 years of age)
• Blood and urine collection of the patient are collected within four days Patients are allowed to be included more than once.

Exclusion Criteria:

• A potential subject who meets any of the following criteria will be excluded from participation in this study:

60 Plasma and urine collections are used of adult patients with serotonin-producing neuro-endocrine tumors.