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Anion Gap

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The anion gap is defined as the difference between the major ECF cation, Na+, and the sum of the major ECF anions, Cl- and HCO3-. Using average normal values for Na+, Cl- and HCO3- {142 – (103 + 24)} gives an anion gap of about 15 mEq/L. The difference is made up of negatively charged proteins, phosphate, sulfate and organic acid radicals such as citrate, lactate, b-hydroxybutyrate and other anions not usually measured in routine blood analyses.

An elevated anion gap is usually associated with metabolic acidosis caused by acid accumulation due to increased metabolic acid production, acid ingestion or acid retention in renal failure. In these cases the anionic component of the acid substitutes for the HCO3- consumed in the buffering process. metabolic acidosis with a normal anion gap is associated with the loss of HCO3- as in diarrhea or renal tubular acidosis in which renal reabsorption and regeneration of HCO3- is inadequate to maintain normal ECF HCO3- levels. In these latter conditions ECF volume contraction occurs due to loss of Na+ with HCO3-. In the presence of low HCO3- concentration in the filtrate and stimulation of Na+ reabsorption, Cl- reabsorption is enhanced, hyperchloremia develops and the anion gap remains about normal.
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