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.

Evaluation of a new system ("Corvac") for separating serum from blood for routine laboratory procedures.

We evaluated the suitability of a new system ("Corvac," Corning) for rapid, complete separation of cell-free serum from whole blood. Twenty-one different clinical chemical procedures were performed on about 1150 paired blood samples, several by more than one method, to afford a more comprehensive evaluation. The tube of clotted blood collected in the conventional manner ("Vacutainer") underwent a double centrifugation and served as the control sample, while its paired specimen collected in the new system was processed by a single centrifugation. We evaluated significant differences between the means of the paired samples, the standard deviations of the differences between the means, and the bias of the means for the values of each constituent for each collection method. For several tests we observed differences that were statistically but not clinically significant. Only in the case of lactate dehydrogenase were large differences consistently demonstrated.