Rapid determination of plasma ammonia using an ion specific electrode

A method for measuring directly the ammonia concentration in plasma by the use of an ammonia gas sensitive electrode is proposed. The plasma pH is brought to a final value of 10.4; at which pH, no NH3 is released from other sources and the amount of NH3 measured is 92% of the total ammonia (pKa = 9.3). The potentiometric readings of the samples are referred to a standard curve. In normal subjects, the mean values of venous ammonia nitrogen are 23.9 ± 9.6 (SD) μg/100 ml of plasma. Reproducibility is ± 0.5 mV, or 2%. The method was also compared to the ion exchange method: The difference was constantly below 3%. The time required for the assay is less than 10 min. The method, when compared to others adopting the same electrode, is recommended for its simplicity, accuracy, and rapidity. © 1975.