The goal of this study is to compare several methods for measurement of Chromogranin A in their ability to serve as a marker for disease activity in patients with neuroendocrine tumors.

Further, in a subgroup, we will determine if taking a proton pump inhibitor affects Chromogranin A levels.

Patient group: patients with neuroendocrine tumors (active or inactive) Healthy volunteers

Patients:

Inclusion Criteria:

• having neuro-endocrine disease

Exclusion Criteria:

• age below 18 or above 70 years
• prostate cancer
• kidney failure (estimated GF < 30 mL/Min)
• heart failure
• chronic atrophic gastritis
• pregnancy

Healthy subjects:

Inclusion Criteria:

• healthy

Exclusion Criteria:

• age below 18 or above 70 years
• taking any chronic medication (except OCP)
• prostate cancer
• kidney failure (estimated GFR < 30 mL/min)
• heart failure
• pheochromocytoma
• islet cell tumors
• medullary thyroid cancer
• essential hypertension
• neurofibromatosis
• use of proton pump inhibitors
• chronic atrophic gastritis
• pregnancy