In screening for pheochromocytoma, which statement about plasma-free metanephrines is true?

• A. They are sensitive to catecholamine release and less affected by renal dysfunction
• B. They require a 24-hour urine collection
• C. They are less specific than plasma metanephrines
• D. They are not affected by any comorbidity

Answer: (A)

Plasma-free metanephrines are a highly sensitive screen for pheochromocytoma because metanephrine and normetanephrine are produced continuously from catecholamines, including within the tumor, so their plasma levels stay elevated even if catecholamine release is episodic. This continuous production makes the test less likely to miss a tumor. They are also relatively less affected by renal dysfunction compared with urinary measurements, since they are measured in plasma rather than relying on renal clearance or a 24-hour urine collection. Therefore, this test is often reliable across different levels of kidney function. The other statements don’t fit: it does not require a 24-hour urine collection; they’re not inherently less specific than other metanephrine assessments; and comorbidity can still influence results, so they aren’t immune to all confounders.