Clonidine Suppression Test
To try and exclude the diagnosis of phaeochromocytoma in patients with hypertension and borderline changes in plasma catecholamines or urinary catecholamine metabolites.
Frail patient with a history of hypotensive episodes or severe coronary or carotid disease.
Hypotension and sedation.
Order the clonidine from pharmacy (readily obtainable).
Stop hypotensive treatment for at least 24 hours before the test if possible.
Sphygmomanometer or Critikon monitor.
Lithium heparin tubes (green top Vacutainers).
Contact biochemistry laboratory who measure catecholamines before doing the test, enquire how they would like the samples taken and arrange for their delivery.
- Insert cannula.
- Rest for 1/2 hour.
- Monitor BP and pulse at onset and every time blood taken.
- Take 2 baseline samples at 5 minute intervals.
- Give, at time 0, 0.3 mg clonidine hydrochloride orally.
- Take blood at hourly intervals for 3 hours.
Clonidine acts via the alpha pre-ganglionic receptors to reduce catecholamine secretion.
In normals, even if they are anxious, the plasma catecholamines will suppress into the normal range 3 hours after clonidine (noradrenaline 0.2-0.8 ng/ml, adrenaline 0.04-2 ng/ml). Phaeochromocytoma patients should not.
SENSITIVITY AND SPECIFICITY
This test gives similar information as the Pentolinium test; there have been no formal comparisons of the two tests. Case reports (see Halter et al., New Engl. J. Med. 306, 49-50 - 1982) have illustrated false negatives. The 24 hr urinary metanephrines/catecholamines have replaced VMAs as the cornerstone of screening for phaeochromocytoma. If a dopamine secreting phaeochromocytoma is suspected on the basis of normo- or hypotension then urinary dopamine and its metabolites should be assayed
Bravo E.L. et al.. New Engl. J. Med. 305, 623-626 (1981)