The clonidine suppression test for pheochromocytoma. A review of its utility and pitfalls.

OBJECTIVE: The intent of this study is to retrospectively evaluate our experience, as well as all published information, regarding the clonidine suppression test to determine its utility, accuracy, and safety in the diagnosis of pheochromocytoma. PATIENTS AND METHODS: All 22 patients (including four with pheochromocytoma) evaluated at a major military referral hospital with the clonidine suppression test for suspected pheochromocytoma for more than 6 years were retrospectively reviewed. All published series of patients similarly evaluated were also critically reviewed.
RESULTS: All studies confirm that a nonstressed plasma norepinephrine of more than 2000 pg/mL is diagnostic of pheochromocytoma. In those patients with a plasma norepinephrine of less than 2000 pg/mL, the clonidine suppression test is 92% accurate in diagnosing pheochromocytoma when the normal response to clonidine is defined as total plasma catecholamines of less than 500 pg/mL. Its accuracy diminishes in patients with low baseline plasma catecholamine levels, who may better be tested with a stimulatory test (ie, glucagon). The use of diuretics, beta-blockers, and antidepressants may cause false-positive results or severe hypotension during the clonidine suppression test. Those previously treated with clonidine or with baroreceptor dysfunction may also be prone to severe hypotension, but this complication is otherwise uncommon after acute clonidine ingestion.
CONCLUSION: Although it is rarely necessary for the diagnosis of pheochromocytoma, the clonidine suppression test is an accurate and safe test in a select group of patients.