Use of sodium concentration and anion gap to improve correlation between serum chloride and bicarbonate concentrations.

Although most acid-base disorders cause opposite and equal changes in serum chloride and bicarbonate concentrations, this inverse relationship can be distorted by changes in the anion gap and/or water balance. Therefore, we examined the relationship between chloride and bicarbonate before and after adjusting for anion gap and serum sodium concentration. Patients with abnormal electrolytes were grouped by chloride and bicarbonate concentrations (low, normal, and high). Then, chloride and anion gap-adjusted bicarbonate were adjusted for water excess (or deficit), manifesting as hyponatremia (or hypernatremia), after which patients were reclassified. Classification by chloride and bicarbonate changed in 82% of the 135 patients after adjustment for anion gap and sodium. Serum chloride and bicarbonate were each low (concordant) in 23 patients, while 18 had discordant chlorides and bicarbonates (9 low/high, 9 high/low). After adjustments, chloride and bicarbonate were discordant in 40 patients (31 low/high, 9 high/low) and concordant in none. The correlation between serum chloride and bicarbonate improved from -0.459 to -0.998 after adjustments for sodium and anion gap. A very close inverse relationship between serum chloride and bicarbonate concentrations is commonly distorted by concomitant water disturbances and anion gap acidoses in internal medicine patients admitted with electrolyte disorders.