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Determining the serum anion gap (the sodium concentration minus the sum of the chloride and bicarbonate concentrations) is helpful in evaluating patients with metabolic acidosis. A generation of clinicians has been taught that the normal anion gap (AG) is 8 to 16 mmol/l. However, since this normal range was established in the 1970s, most laboratories have changed their methods for measuring serum electrolytes, with a resulting downward trend in the AG.
To determine a more accurate modern reference range for the AG, Winter and colleagues measured electrolytes in 149 healthy subjects and in reference specimens from the National Bureau of Standards, using an Automated Stat/Routine Analyzer (ASTRA) in three hospital laboratories. They found that…