What are the principles of biochemical testing?
When a hormonally active tumor is suspected, the diagnosis should be confirmed biochemically before any imaging is performed. This is important not only for cost-effectiveness but also because some localization studies are invasive.
For most biochemical studies an 8-hour fast is recommended. It is important to recognize that certain medications can affect test results, especially proton pump inhibitors (PPIs), which significantly increase serum gastrin and chromogranin A levels.
Patients with suspected insulinoma should undergo a 72-hour fast with serial measurements of glucose, serum insulin, proinsulin, C-peptide, and sulfonylurea screening.
Evaluation for possible ZES should include measurements of fasting serum gastrin levels and gastric pH. Those suspected of having a glucagonoma, VIPoma, or somatostatinoma should have measurements of serum glucagon, VIP, and somatostatin levels, respectively.
If a familial syndrome is suspected, screening for other syndromic components should be undertaken, and genetic testing should be considered. For example, if MEN 1 is being considered, patients should be evaluated for coexisting hyperparathyroidism and pituitary tumors, and genetic testing for the menin gene mutation is usually appropriate.