title: Approach to the Patient With a Negative Anion Gap.
abstract: When anion gap calculation generates a very small or negative number, an explanation must be sought. Sporadic (nonreproducible) measurement errors and systematic (reproducible) laboratory errors must be considered. If an error is ruled out, 2 general possibilities exist. A true anion gap reduction can be generated by either reduced concentrations of unmeasured anions such as albumin or increased concentrations of unmeasured cations such as magnesium, calcium, or lithium. This teaching case describes a patient with aspirin (salicylate) poisoning whose anion gap was markedly reduced (-47 mEq/L). The discussion systematically reviews the possibilities and provides the explanation for this unusual laboratory result.
authors.affiliation: Nephrology Division, Department of Internal Medicine, Baylor University Medical Center, Dallas, TX. Electronic address: m.emmett@baylorhealth.edu.
authors.forename: Michael
authors.initials: M
authors.lastname: Emmett
mesh_terms: D000137:Acid-Base Imbalance / Q000139:chemically induced / Q000175:diagnosis / Q000628:therapy; D001241:Aspirin / Q000506:poisoning; D005260:Female; D006801:Humans; D008875:Middle Aged; D013406:Suicide, Attempted
chemical_list: D001241:Aspirin
keywords: Low anion gap; aspirin toxicity; bromism; negative anion gap; pseudohyperchloremia; salicylate poisoning