What is the first step in diagnosing pheochromocytoma?

The first step in diagnosing pheochromocytoma is checking plasma free metanephrines. This is because pheochromocytoma is a tumor that secretes catecholamines, leading to elevated levels of their metabolites, metanephrines, in the bloodstream. The measurement of plasma free metanephrines is a sensitive and specific test that can help confirm the diagnosis of pheochromocytoma even when catecholamine levels might be fluctuating or normal during the testing period.

In clinical practice, measuring plasma free metanephrines is often preferred over imaging studies or other laboratory tests as an initial screening tool due to its non-invasive nature and the fact that it detects abnormalities indicative of the presence of the tumor. If the plasma free metanephrine test is positive, it can then guide further diagnostic imaging, such as a CT scan of the abdomen, to localize the tumor.

Serum electrolytes and blood glucose tests do not provide specific information related to pheochromocytoma and are not useful as initial diagnostic steps.