Patient Preparation: The patient should
(1) be resting and not be stressed
(2) not be receiving medications
(3) not have received contrast material recently.
Specimen: A 24 hour urine is preferred. Testing can be done on urines collected for shorter periods, but a short collection period can miss intermittent release of catecholamines.
urinary metanephrine-to-creatinine ratio =
= (output of metanephrine in micrograms per time period) / (output of creatinine in milligrams per time period)
Interpretation:
• A ratio > 2.2 micrograms of metanephrine to milligram creatinine suggests pheochromocytoma
• The results are sensitive but may not be specific for pheochromocytoma
Limitations:
• Many drugs can cause false positives.
• Stress can cause false positives.
• Patients with hypertension from other causes can have false positive results.
• imaging contrast material can cause false negatives.
• Since excretion of catecholamines from a pheochromocytoma may be episodic, a spot urine may miss the increased release.
