Determining Reference Ranges by Linear Analysis: Serum Electrolyte Concentrations

Abstract

We determined reference ranges for serum sodium, potassium, and chloride concentrations by collecting results from 200 consecutive hospitalized patients. For each electrolyte, the value at each whole percentile was determined, and the value-vs-percentile plot was constructed. The central linear portion of each value-vs-percentile plot was determined as follows: First, the least-squares straight line with 95% confidence limits was calculated through all values. Second, the highest values above the high confidence limit and lowest values below the low confidence limit were excluded. Third, the least-squares straight line with 95% confidence limits was recalculated through all unexcluded values. Fourth, extreme values again were excluded as described above, and the process was repeated until highest and lowest values were within the 95% confidence limits. The line connecting these extreme values was extended to the Oth and 100th percentiles, and the values at these percentiles were defined as the reference limits.