False Positive Reaction Due to Methocarbamol in the Screening Test for Vanilmandelic Acid (VMA)

Methocarbamol, a widely prescribed muscle relaxant, gives a positive result for increased vanilmandelic acid excretion with the useful screening test devised by Gitlow for evidence of pheochromocytoma. Evidence for the presence of this drug can be obtained from a positive color reaction with the 5-hydroxyindoleacetic acid test, used in the screening for carcinoid tumor. I N PHEOCHROMOCYTOMA, 3-methoxy-4-hydroxy-rnandelic acid (VMA), a metabolite of epinephrine and norepinephrine, is excreted in urine in abnormally large quantities. Qitlow et al. (1) have developed a semi quantitative screening method for VMA that we have found to be use-ful and reliable. The procedure is particularly welcome because it per-mits screening of a large number of urines from hypertensive patients and is much more convenient than the measurement of catecholamines. Also, a reagent kit based on Gitlow’s colorimetric reaction is commer-cially available. * We have performed more than 400 such urine tests and 2 patients in whom the tests indicated increased YMA were sur-gically proved to have pheochromocytomas. Briefly, the test as described by Gitlow involves extraction of the acidified urine with ethyl acetate, evaporation of the solvent, and solu-tion of the residue in dilute K2C03. Diazotized p-nitroaniline is added and the mauve-colored azo-VMA product is extracted into amyl alco-hol made alkaline with ethanolamine. Two readings are made in