Vanillylmandelic acid, or VMA, is a substance found in the urine of a child who has a type of abdominal tumor called a neuroblastoma. The VMA test was once used to diagnose another type of tumor – pheochromocytoma – but is now a secondary test for that condition. You must restrict your diet for several days prior to a VMA test to prevent false positives.
VMA Test
The VMA test measures the amount of VMA in the urine. Your doctor might use an initial VMA test to diagnose or rule out a neuroblastoma – an abdominal tumor usually found in children under age 5. Other possible tests with the VMA include a urine creatinine and an HVA, or homovanillic acid test. Neuroblastomas and pheochromocytomas cause excess secretion of substances called catecholamines. These lab tests help to show if your body has high levels of catecholamines.
Test Sensitivity
The VMA test is sensitive to outside substances and even physical or emotional stress. This sensitivity increases the risk that a positive result for a VMA test is actually a false positive rather than a true reflection of VMA status. The presence of a high VMA level is not considered diagnostic, but is an indication that there must be further investigation to obtain a correct diagnosis.
False Positives
The VMA test is performed on a single urine sample or after a 24-hour urine collection. Physicians sometimes advise patients to restrict the use of some prescription drugs as well as certain foods before a VMA test. Beverages that contain caffeine may increase secretion of catecholamines and lead to a false positive. Foods that contain vanilla may also increase false positives. Sometimes, doctors order other catecholamine tests on the same urine sample collected for the VMA test; these tests may require slightly different food restrictions.
Diet Restrictions
Medline Plus recommends that patients avoid coffee, tea, bananas, chocolate, cocoa, citrus fruits and vanilla for several days before the test. The United States Army Medical Department recommends that you completely avoid bananas, caffeine, chocolate, vanilla, citrus fruits, tomato, pineapple, alcoholic beverages, avocados, eggplant, plums and walnuts. MayoClinic.com does not recommend dietary restrictions. Follow the instructions of the health-care professional who orders your tests.
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