Title: More on the Inhibition of the Urease Reaction by Boric Acid
Abstract: Boric acid has been used for many years as a urine preservative for microbiological tests! and for measurement of urinary urea.? It has, however, been reported that boric acid can interfere with the analysis of urea by methods which utilize urease as a reagent. 3 I have investigated the effect of boric acid on two urease-based assays and confirmed that boric acid interferes with these assays. I report a difference in sensitivity to boric acid between the two methods, and discuss possible reasons for this difference. In addition I report that this interference may occur at a lower concentration of boric acid than has been reported previously. A 24 h urine collection was received for measurement of urea and other analytes from a patient who was being treated with peritoneal dialysis for renal failure. The sample volume was 450 mL. The collection bottle had been supplied to the patient by a general practitioner in a rural location. The urea concentration was measured in various dilutions on a Beckman eX7 analyser (Beckman Instruments Inc., USA) and also on a Boehringer-Mannheim Hitachi 747 analyser (Boehringer-Mannheim GmBH, Germany) using the reagent kit supplied by the manufacturers. Non-linear dilution patterns were seen which differed between the two instruments (Fig. la). Subsequent enquiries revealed that the collection bottle contained boric acid. The eX7 and 747 analysers both use Jack Bean urease for the measurement of urea concentrations. The eX7 analyser uses a one in 100 dilution of the sample and monitors the urease reaction by measuring the increase in conductivity of the solution II s after the addition of the enzyme. The 747 analyser measures the ammonium produced by the urease reaction with an auxiliary enzyme system consisting of NADH, glutamate dehydrogenase