Chromogranin A is the first line test for diagnosing Carcinoid tumors. It is used as a followup test for treated cases of Carcinoid tumors. It is useful as an adjunct in the diagnosis of other Neuroendocrine tumors including Pheochromocytoma, Pituitary adenomas and functioning & non-functioning Islet cell and Gastrointestinal APUD tumors.