Plasma Electrolyte Distributions in Humans—Normal or Skewed?

Abstract

BackgroundIt is widely believed that plasma electrolyte levels are normally distributed. Statistical tests and calculations using plasma electrolyte data are often reported based on this assumption of normality. Examples include t tests, analysis of variance, correlations and confidence intervals. The purpose of our study was to determine whether plasma sodium (Na+), potassium (K+), chloride (Cl−) and bicarbonate (HCO3−) distributions are indeed normally distributed. MethodsWe analyzed plasma electrolyte data from 237 consecutive adults (137 women and 100 men) who had normal results on a standard basic metabolic panel which included plasma electrolyte measurements. The skewness of each distribution (as a measure of its asymmetry) was compared to the zero skewness of a normal (Gaussian) distribution. ResultsThe plasma Na+ distribution was skewed slightly to the right, but the skew was not significantly different from zero skew. The plasma Cl− distribution was skewed slightly to the left, but again the skew was not significantly different from zero skew. On the contrary, both the plasma K+ and HCO3− distributions were significantly skewed to the right (P < 0.01 zero skew). There was also a suggestion from examining frequency distribution curves that K+ and HCO3− distributions were bimodal. ConclusionsIn adults with a normal basic metabolic panel, plasma potassium and bicarbonate levels are not normally distributed and may be bimodal. Thus, statistical methods to evaluate these 2 plasma electrolytes should be nonparametric tests and not parametric ones that require a normal distribution.