Inadequate measurement of urinary chloride.

Urinary electrolytes are invaluable in the differential diagnosis and treatment of certain acid-base diseases. We have evaluated the accuracy and precision of three different automated chemistry analyzers and a chloridometer (Corning 925 Chloridometer, Hitachi 717 and 917 and Vitros 950) for chloride, sodium and potassium using standard solutions. Data indicate that the ISE modules on the Hitachi 717 and 917 analyzers measure sodium and potassium accurately and precisely throughout the studied concentration range. The Vitros 950 system had the poorest precision and accuracy performance at the low levels of sodium and potassium. The chloride ion-selective membranes on the Hitachi analyzers seriously overestimate the chloride concentration at all levels. The manual dilution method on the Vitros 950 analyzer for chloride measurements showed good precision and accuracy only at the higher chloride levels. For chloride determination the chloridometer displayed the best accuracy with good precision. We recommend that laboratories use chloridometers for the measurement of urinary chloride, and that the CAP include low chloride samples in their proficiency testing program for urine chloride.