The results of plasma-ammonium determinations by an ion-exchange method and by a microdiffusion method were compared on the same specimens with each other and with ammonium determinations carried out on the corresponding whole-blood specimens by the microdiffusion method. The plasma-ammonium levels of whole-blood specimens were higher than those of the corresponding plasma specimens, but the difference was not constant. Recoveries of added NH4 from plasma by the microdiffusion method ranged from 65 to 144%. Recoveries of NH4 from the same specimens by the ion-exchange method ranged from 93 to 96%. Of the two, the ion-exchange method was the only one of value.
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