Evaluation of an instrument (Nova-1) for direct potentiometric analysis of sodium and potassium in blood and their indirect potentiometric determination in urine

We evaluatede and compared the Nova-1 to an alternative direct potentiometric analyzer (Orion-SS/30) and to flame photometry (Beckman 'KLiNa Flame'). Interassay precision of the Nova-1 for plasma was better than flame photometry but slightly worse than the Orion SS/30. For urine, interasay precision of the Nova-1 was not as good as for the flame photometer, but adequate for clinical use. Calcium, magnesium, phosphate, pH, uric acid, creatinine, and urea influenced neither sodium nor potassium values. Ammonia did not interfere with blood analysis but could slightly increase potassium values in urine when a maximum acid load is excreted. Unlike flame-photometric values, increasing protein concentrations did not influence the Nova-1 values. Whole blood and plasma gave virtually identical values. In plasma, the Nova-1 gave slightly lower values for sodium (0.8 mmol/L) and potassium (0.016 mmol/L) than the Orion SS/30. Like the Orion SS/30, the Nova-1 gave higher values than flame photometry for both sodium (3.9 mmol/L) and potassium (0.07 mmol/L). Urine samples showed good agreement between the Nova-1, operated in the urine mode, and flame photometry for sodium and for potassium if samples with electrolyte values higher than 50 mmol/L were analyzed after dilution with de-ionized water. Evidently, the Nova-1 is precise, capable of accurately measuring sodium and potassium in whole blood, plasma, or urine (with the above restrictions), and should be more accurate, as is the Orion SS/30, than flame photometry for assaying specimens with above-normal protein or lipid values.