Automated enzymatic method for determining ammonia in plasma, with 14-day reagent stability.

This is an automated, cost-efficient enzymatic method for determining ammonia in plasma. In most assays for ammonia the reagents are stable for only one or two days at 4 degrees C, but the reagents in our method are stable for at least 14 days. Our method was developed for use with a Cobas-Fara analyzer but can be easily adapted to other automated analyzers. It is essential to use closed sample vials. Our results, obtained at much less cost, correlate well with those by the Du Pont aca method. Precision studies gave the following results: within-run (n = 20 each), for x = 46 mumol/L, SD = 0.7 mumol/L, CV = 1.6%, and for x = 593 mumol/L, SD = 4.0 mumol/L, and CV = 0.7%; day-to-day precision during a year: for x = 47.7 mumol/L, SD = 2.4 mumol/L, CV = 5.0% (n = 360); and for x = 643 mumol/L, SD = 20.5 mumol/L, and CV = 3.2% (n = 365). The standard curve was linear to 1000 mumol of NH3 per liter.