Lack of suppression of NM more than 40 % and NM above upper limit of normal (Sensitivity (sn): 96 %, specificity (sp):100 %)

Elevated plasma concentration of NM above upper normal reference range (sn: 96 %, sp: 96 %)

Elevated plasma concentration of NE above upper normal reference range (Sn: 71 % and sp: 94 %)

Less than 50 % NE suppression (sn 81 %, sp: 82 %)

The test may be associated with hypotension in patients with normal blood pressure.

In most patients careful evaluation of plasma and urinary metanephrines and catecholamines of interfering drugs, and their correlation with imaging studies (if available) may obviate the need for a clonidine suppression test .

Medications to avoid prior to clonidine suppression test: phenoxybenzamine and tricyclic antidepressants. Selective Alpha 1 -adrenergic blockers do not interfere with the test and can be used to control hypertension [3–4]. Beta blockers, calcium channel blockers, and diuretics may affect plasma norepinephrine levels, but do not have any significant effect on normetanephrine levels [3–4].

Patients who have relatively low baseline catecholamines levels (< 1000 pg/ml) may have high false positive test results if criteria less than 50 % reduction on norepinephrine levels is used [3, 5].

Measurement of plasma normetanephrine levels during clonidine suppression test provide better diagnostic sensitivity and specificity compared to plasma norepinephrine levels [3].

The clonidine suppression test cannot be used to evaluate patients with an elevated plasma metanephrine fraction since clonidine has a minimal effect on metanephrine levels. Similarly, the clonidine suppression test cannot be used in patients with rare dopamine secreting pheochromocytomas or measurement of plasma methoxytyramine (when commercially available); plasma or urinary dopamine may be used in such patients.