Final answer:
The first line investigation for a pheochromocytoma diagnosis involves measuring plasma or urine levels of catecholamines and their metabolites, especially metanephrines. Following positive results, imaging tests are performed to localize the tumor.
Step-by-step explanation:
The first line investigation for diagnosing pheochromocytoma, a tumor of the adrenal medulla causing excessive production of catecholamines and resulting in hypertension, typically starts with non-invasive tests. It is essential to measure the levels of catecholamines and their metabolites, specifically metanephrines, in the patient's plasma or urine. The most commonly used initial tests are the plasma free metanephrines or the 24-hour urine collection for fractionated metanephrines and catecholamines. These tests have a high sensitivity for detecting pheochromocytoma. In case of elevated metanephrines, further imaging studies, like CT, MRI of the abdomen, or meta-iodobenzylguanidine (MIBG) scanning, are used to localize the tumor.