Type of test Blood

Normal findings

 Glucagon stimulation  Norepinephrine: < 3 times basal levels

Clonidine suppression Norepinephrine: > 50% reduction in basal levels or < 500 pg/mL

Epinephrine: > 50% reduction in basal levels or < 275 pg/mL

 Normetanephrine > 40% reduction in basal levels.

Test explanation and related physiology

In patients with significantly high blood pressure refractory to treatment, the diagnosis of pheochromocytoma or paraganglioma are often considered. When catecholamine levels are excessive (norepinephrine > 2000 pg/mL), the diagnosis is easily made. However, when basal levels are not significantly elevated, it is difficult to differentiate essential hypertension (or other causes of elevated catecholamines) from a functioning pheochromocytoma. Suppression and provocative tests may be necessary after repeating the testing of blood or urine for other catecholamines. Glucagon can be used as the provocative agent. In patients with pheochromocytoma, the response is accentuated. However, glucagon stimulation offers insufficient diagnostic sensitivity for reliable exclusion or confirmation of pheochromocytoma. Because of this and the risk of hypertensive complications, the test is not routinely used. More commonly biochemical and radiologic testing is used instead of provocative testing. Clonidine is normally a potent suppressor of catecholamine production, yet it has little to no effect on catecholamines in patients with pheochromocytoma. Testing of metanephrines provides higher diagnostic sensitivity than catecholamine assays in screening for pheochromocytoma. In suppression testing, normetanephrine is the most accurate biochemical to test.

Contraindications

• Hypovolemic/dehydrated patients. These patients should not have suppression testing because they could experience a precipitous drop in blood pressure.

Potential complications

• Drowsiness during CST

• Hypotension during CST, especially in patients treated aggressively for hypertension

 • Extremely high blood pressure during provocative testing

Interfering factors

• False suppression with CST may occur in patients with low basal catecholamine levels.

Procedure and patient care

 Before

* Explain the procedure to the patient.

 • Identify the medications being administered before testing.

 • The patient must be reclining calmly for 30 minutes before testing.

• The patient must be fasting for at least 10 hours.

 During

 • Collect a venous blood sample from an antecubital vein in a heparinized tube for determination of basal catecholamine levels.

• Monitor vital signs closely throughout the testing period.

Glucagon provocative test

• Administer a prescribed dose of glucagon intravenously.

 • Two minutes later, obtain a blood specimen as described above.

 Clonidine suppression test

 • Administer a prescribed dose (usually 0.3 mg for normal sized adults) of clonidine orally.

 • Three hours later, obtain a blood specimen as described earlier. Place specimen in ice.

After

 • Monitor vital signs for at least 1 hour after conclusion of the procedure.

 Abnormal findings

- Pheochromocytoma

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