Which test should be used to help confirm pheochromocytoma?

• A. Serum cortisol
• B. Plasma metanephrines
• C. Fasting glucose
• D. 24-hour urine vanillylmandelic acid

Answer: (D)

Excess catecholamines produced by a pheochromocytoma are metabolized to metanephrines and to vanillylmandelic acid (VMA). Measuring the amount of VMA excreted in a 24-hour urine collection provides an integrated snapshot of catecholamine turnover over a full day, which helps confirm persistent excess rather than a single episodic spike. This test has long served as a confirmatory biochemical test for pheochromocytoma after an initial screen suggests excess catecholamines or metanephrines.

Keep in mind that while useful for confirmation, 24-hour urinary VMA is less sensitive than tests that directly measure metanephrines (like plasma free metanephrines or 24-hour urinary metanephrines) and can be influenced by dietary factors and medications, as well as by incomplete urine collection. In modern practice, plasma free metanephrines are often the preferred screening test, with 24-hour urinary VMA serving as a historical or confirmatory option in certain settings.