The automated thiosemicarbazide-diacetyl monoxime method for plasma urea.

IN RECENT YEARS the automated diacetyl monoxime (DAM) reaction as adapted to the AutoAnalyzer has become the method of choice in many clinical laboratories for the determination of urea in biologic fluids. The method is based on the condensation of urea with diacetyl, the latter usually being liberated from the monoxime in the presence of one of various acid reagents (1, 2). The reaction, however, does not follow Beer’s law, and the colored product has been reported to be photosensitive (3, 4). By the addition of thiosemicarbazide (TSC) to the diacetyl reagent, Coulombe and Favreau (5) obtained linearity of response with a stable pink color whose Emat was shifted to 535 nm; the mixed reagent was reportedly stable at room temperature. Pellerin (6) automated the method, reporting exact linearity up to concentrations of 50 mg/100 ml. Marsh et al (7) compared manual versus automated methods employing TSC. In this comparison they modified both acid and DAM reagents somewhat, noting that color intensity of the product formed was proportional to concentrations of acid, DAM, and (to an optimum for any given mixture) TSC; linearity was not mentioned. We have confirmed the general observations of the latter authors but found that the recommended composition of acid reagent (0.05% w/v FeCI3 in a mixture of 8% 1)y volume of concentrated sulfuric and 1% by volume phosphoric acid) yielded very low color intensities, which could be increased many times over by using more conventional concentrations of ferrie alum acid reagent. For automatic logging and computation in a multicliannel system, our requirements were precise linearity over the concelltration range