The anion gap (AG) in the serum equals the concentrations of Na - (Cl + HCO3). It is becoming increasingly useful in the interpretation of acid-base disorders and in the diagnosis of other conditions. In an acidemic patient, an elevated AG usually indicates the presence of an organic acidosis, rhabdomyolysis, nonketotic hyperglycemic coma, uremia, or certain intoxications. An increased AG with alkalemia suggests severe alkalosis with hemoconcentration or use of anionic antibiotics (eg, carbenicillin) or salts of organic acids (eg, citrate). An elevated AG with a normal serum pH could be an artifact caused by prolonged exposure of the serum sample to air before processing. A decreased AG with a normal serum pH may indicate hypoalbuminemia, cationic paraproteinemia, halide poisoning, or lithium intoxication. The ΔAG/ΔHCO3 ratio and the urinary AG may also be quite useful in analyzing complex acid-base disorders.
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