Development and evaluation of a kinetic diacetyl monoxime method for urine urea

A kinetic assay for urine urea which employs the reaction with diacetylmonoxime at 37 degrees C and undiluted urine was developed. Reaction conditions were optimized using the Monarch centrifugal analyser to produce an assay with the linearity to 450 mmol/L urea, and CVs of 2.3% and 2.5% within-batch and 4.6% and 3.3% between-batch at urea concentrations of 75.9 and 162.7 mmol/L, respectively; recoveries of 98.5% to 99.5% were obtained with the addition of the equivalent of 95 and 190 mmol/L urea. Comparisons were made with one urine urea method employing diacetylmonoxime with dialysis and two enzymatic methods which all involve a sample pre-dilution stage. The urine samples used for the method comparisons did not contain preservatives. However, a range of urine preservatives and a selection of possible interfering metabolites were shown to have no significant effect on the recovery of urea in the assay.