Patient Preparation:
The patient is fasted overnight. In the morning the patient is made recumbent and is kept lying during the entire procedure. An indwelling venous catheter is inserted and baseline blood samples are drawn. Clonidine at a dose of 4.3 µg per kilogram of body weight is then given orally.
Specimen Collection:
(1) Serum for baseline levels after the patient has been recumbent for 30-60 minutes
(2) 3 hours after clonidine administration
Testing: Norepinephrine levels are measured in each of the samples.
Interpretation:
• Normal persons will show plasma norepinephrine levels at 3 hours after clonidine administration within the normal reference range (150-650 ng/L), usually below 500 ng/L.
• Plasma norepinephrine levels that exceed the reference range is supportive for the diagnosis of pheochromocytoma.
Performance:
• False positive results may be seen with physical stress, exercise, mental stress, smoking, assuming a sitting or standing position and chronic illness.
• False negative results may occur if release of catecholamines from the pheochromocytoma is intermittent rather than continuous.
