Plasma Electrolyte Distributions in Humans—Normal or Skewed?

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It 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 Symbol distributions are indeed normally distributed.


We 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.


The 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 Symbol 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 Symbol distributions were bimodal.


In 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.

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