Apparent Positive Interference from an Etoposide Metabolite, but Not Etoposide, in Measuring Urinary Vanillylmandelic Acid

For 15 years, we have determined urinary vanillylmandelic acid (VMA) by a method (1) that relies on the reaction of extracted VMA with the diazo derivative of p -nitroaniline to give a pink chromophore; the chromophore undergoes a bathochromic shift in the presence of an aprotic solvent and a base to give a blue derivative with maximum absorbance at 600 nm. Hitherto, we observed neither positive nor negative interferences.Recently, however, we observed the development of a pink color in the final stage of the procedure, masking the blue color of VMA (Fig. 1⇓ ). The urine …