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Monday, August 16, 2010
This 24-hour urine test for VMA and catecholamines is performed primarily to diagnose hypertension secondary to pheochromocytoma. This test is also used to detect the presence of neuroblastomas and rare adrenal tumors.
A 24-hour urine test is preferable to a blood test because catecholamine secretion from the tumor may be episodic and could potentially be missed at any one time during the day. The urine provides the laboratory with a specimen that reflects catecholamine production over one whole day. It is best to perform testing when the symptoms of the potential adrenal tumors are significant. At that time, catecholamine production is the greatest and will be more assuredly identified.
A pheochromocytoma is an adrenal tumor that frequently secretes abnormally high levels of epinephrine and norepinephrine. These hormones cause episodic or persistent hypertension by causing peripheral arterial vasoconstriction. Dopamine is the precursor of epinephrine and norepinephrine. Metanephrine and normetanephrine are catabolic products of epinephrine and norepinephrine, respectively. VMA is the product of catabolism of both epinephrine and norpeinephrine. In patients with pheochromocytoma, one or all of these substances will be present in excessive quantities in 24-hour collection of urine.
Sample Collection and Stability
Urine specimens for assessment of 24-hour output of catecholamines should be collected on ice in a container with 10 mL added HCl. During the period of sample collection, the urine should be refrigerated to minimize degradations of materials. After the collection is made, the pH should be adjusted to somewhere between pH 2 and 5 with 6M HCl. Aliquots can be frozen indefinitely, but repeated thawing and refreezing should be avoided.
Analysis of Urine Cateholamines
- Urine catecholamines: Fluorometric Assay
Epinephrine and norepinephrine excretion in urine can be measured in a variety of ways. These compounds are excreted both in the free and conjugated states. Most clinical work has been done using data on free cateholamines. Inclusion of an initial acid hydrolysis step allows the assay of total epinephrine and norepinephrine using the same methods employed for the quantitation of the free fractions.
- Urine Metanephrine: Colorimetric Assay
Total metanephrines can be analyzed by the Pisano Method, which involves extraction followed by colorimetric reaction. This assay does not distinguish between metanephrine and normetanephrine but gives the total of the two components.
- High-Performance Liquid Chromatographic Determination of Catecholamines
The high resolution power oh HPLC has been successfully adapted to the quantitative measurement of catchoalamines in a variety of situations. Methods are available for fractionation of both plasma and urine catecholamines in the clinical laboratory. Research applications includes the study of catecholamine levels in various tissues, providing invaluable information about the details of catecholamine metabolism and the changes seen in various pathological condition.
Adolescent: 1-5 mg/24 hr
Child: 1-3 mg/24 hr
Adult/Elderly: 20mcg/ 24 hr
0-1 year: 0-2.5 mcg/24 hr
1-2 years: 0-3.5 mcg/24 hr
2-4 years: 0-6 mcg/24 hr
4-7 years: 0.2-10 mcg/24 hr
7-10 years: 0.5-14 mcg/24 hr
Adult/Elderly: 100mcg/ 24 hr
0-1 year: 0-10 mcg/24 hr
1-2 years: 0-17 mcg/24 hr
2-4 years: 4-29 mcg/24 hr
4-7years: 8-45 mcg/24 hr
7-10 years: 13-65 mcg/24 hr
Adult/Elderly: 65-400 mcg/ 24 hr
0-1 year: 0-85 mcg/24 hr
1-2 years: 10-140 mcg/24 hr
2-4 years: 40-260 mcg/24 hr
15-80 mcg/24 hr or 87-473 nmol/day (SI units)
Clinical Chemistry: A fundamental Textbook by Donald F. Calbreath, PhD