MEASUREMENT OF PLASMA METHOXYAMINES - A PRACTICAL ADVANCE IN THE DIAGNOSIS OF PHEOCHROMOCYTOMA

Measurement of plasma methoxyamines is aimed to provide a reliable pla sma marker readily available for any patient suspected of having a phe ochromocytoma. The present-HPLC method is able to detect methoxyamine amounts as low as 0,2 nmoles/l and allows the determination of both fr ee or conjugated methoxyamines (metanephrine MN and normetanephrine NM N) in normal subjects, treated or untreated patients and a fortiori in pheochromocytoma. The analytical specificity is excellent. Among most usual antihypertensive drugs, only conversion enzyme inhibitors and d iuretics induce in some patients a moderate increase of NMN with minor consequence in values interpretation. The kidney plays a major role i n the blood clearance of methoxyamines and any alteration of renal fun ction is associated to the increase of plasma methoxyamine levels. Pla sma methoxyamines have a long half-life and are long-lasting integrate d markers of catecholamine secretion; at variance with plasma free cat echolamines, they are able to afford the presence of a secreting pheoc hromocytoma whatever the clinical presentation - asymptomatic or parox ysmal form between the crisis. The diagnostic sensitivity (at least 98 %) is equal to that of urinary methoxyamines, far higher than plasma or urine catecholamines or urine VMA (60 to 70 %).