Performance of the Du Pont aca ammonia method

We evaluated the performance of the Du Pont aca ammonia procedure with regard to (a) linearity, (b) precision, (c) interferences, and (d) effect of anticoagulants. Linearity extends to 2,000 mumol/L. The SD of the method was essentially constant (2 to 3 mumol/L) and independent of the NH3 concentration. Hemoglobin, bilirubin, and lipemia do not interfere. Either EDTA or heparin is suitable as anticoagulant. Recovery of NH3 added to plasma samples averaged 102% (range: 97--107%). We established normal values by measuring NH3 in 188 plasma samples from apparently healthy individuals. The 95% confidence range is from 10 to 35 mumol/L. The aca ammonia method compares very well with that of Kingsley and Tager but correlates less strongly with that of Reinhold and Chung. We describe a protein-based solution with stable NH3 concentration that is suitable as a control material.