HPLC for urinary catecholamines and metanephrines with alpha-methyldopa

Area of Science:

• Clinical Chemistry
• Analytical Chemistry
• Pharmacology

Background:

• The trihydroxyindole (THI) fluorometric assay for urinary catecholamines and metanephrines is prone to false-positive results.
• Alpha-methyldopa, an antihypertensive, interferes with the THI assay due to structural similarities with endogenous catecholamines.

Purpose of the Study:

• To evaluate high-performance liquid chromatography (HPLC) with electrochemical detection for accurate quantitation of urinary catecholamines and metanephrines.
• To compare the clinical utility of HPLC with the conventional THI-fluorometric method during alpha-methyldopa administration.
• To establish HPLC as a reliable diagnostic tool for pheochromocytoma in patients taking alpha-methyldopa.

Main Methods:

• High-performance liquid chromatography (HPLC) with electrochemical detection was used to measure urinary catecholamines and metanephrines.
• The HPLC method was validated in healthy volunteers and a patient with pheochromocytoma receiving alpha-methyldopa.
• Results were compared against the conventional trihydroxyindole (THI)-fluorometric assay.

Main Results:

• HPLC provided accurate and reproducible results, unaffected by the presence of alpha-methyldopa.
• Unlike the THI method, HPLC did not show elevated excretion rates of epinephrine, norepinephrine, and dopamine in the presence of alpha-methyldopa.
• HPLC indicated a slight decrease in normetanephrine excretion with alpha-methyldopa, contrasting with the THI method's slight increase. The study also suggested the formation of a normetanephrine analog of alpha-methyldopa.

Conclusions:

• HPLC is a suitable and accurate method for quantifying urinary catecholamines and metanephrines, even in patients treated with alpha-methyldopa.
• HPLC overcomes the interference issues associated with the THI assay, providing reliable results for pheochromocytoma diagnosis.
• The use of HPLC eliminates the need to discontinue alpha-methyldopa therapy for pheochromocytoma screening.