Effect of sodium on the auto analyzer flame photometric determination of potassium in serum.

Abstract

Apparent inaccuracies in serum potassium determinations have been detected consistently by our quality control program. These inaccuracies are caused by interference of Na+ in the K+ determination, and result from differences in Na+/K+ ratios of standards and controls (or specimens). Determined K+ values can be corrected by simple calculations if the differences between Na+ levels of standards and specimens are known. Accurate K+ values can also be obtained by using “radiation buffers,” and expected K+ values are obtained when standards and controls both contain 1,500 mEq. per 1. Na+ or 1,000 mEq. per 1. Cs+. Dilution with methanol (80:20 v/v) is equally effective in eliminating interference from sodium. Commercial abnormal control sera and aqueous standards have a variety of high–high, high–low, and low–low mixtures of Na+ and K+, respectively, even though statistical analyses of large numbers of random clinical specimens do not show any correlation between Na+ and K+. The widespread use of diverse standards undoubtedly contributes significantly to interlaboratory variance observed in laboratory surveys.