Should Doxazosin Be Stopped Before Measuring Urine Metanephrines?
No, doxazosin does not need to be stopped before measuring urine metanephrines, as common antihypertensive medications including alpha-blockers do not interfere with metanephrine measurements when using modern analytical methods. 1
Evidence Supporting Continued Use During Testing
Modern immunoassay methods for urinary catecholamines and metanephrines are free from drug interference, including from antihypertensive medications, making it unnecessary to discontinue these drugs before testing 2
The American College of Cardiology states that common antihypertensive medications do not affect plasma free metanephrine measurements when using LC-MS/MS analysis 1
A comprehensive study evaluating new immunoassay methods found no significant differences in free catecholamine or total metanephrine concentrations between patients taking various medications and control groups receiving no medication (P = 0.649 for free epinephrine, 0.221 for free norepinephrine, 0.149 for total metanephrine, and 0.170 for total normetanephrine) 2
Clinical Context for Doxazosin Use
Doxazosin is an alpha-1 selective blocker commonly used in the preoperative management of pheochromocytoma patients, typically started 7-14 days before surgery with gradually increasing dosages until blood pressure targets are achieved 3
In patients with confirmed or suspected pheochromocytoma, doxazosin has demonstrated excellent antihypertensive efficacy (79.2% overall, 91.7% when combined with beta-blockers) and importantly, urinary and plasma catecholamine levels tended to decrease or remain unchanged during doxazosin therapy 4
Important Caveats About Drug Interference
The notable exception is labetalol (a combined alpha- and beta-blocker), which produces false elevations in urinary catecholamine and metanephrine measurements and should be discontinued at least 3 days before testing 5
Several other foods and medications may interfere with metanephrine analysis and should be avoided prior to testing, but alpha-1 selective blockers like doxazosin are not among them 3, 1
False positive elevations in hypertensive patients are usually less than 4 times the upper limit of normal and are more commonly associated with obesity, obstructive sleep apnea, or tricyclic antidepressants rather than alpha-blockers 1
Practical Testing Recommendations
Confirm that interfering agents (excluding doxazosin) were avoided prior to testing 3, 1
For plasma free metanephrines, ideally collect from an indwelling venous catheter after the patient has been lying supine for 30 minutes to limit false positive results 3, 1
Both plasma free metanephrines (sensitivity 96-100%, specificity 89-98%) and 24-hour urinary fractionated metanephrines (sensitivity 86-97%, specificity 86-95%) are acceptable first-line tests 1, 6