Uses
Anion gap is an 'artificial' and calculated measure that is representative of the unmeasured ions in plasma or serum (serum levels are used more often in clinical practice).
Commonly measured cations include sodium (Na+), Potassium (K+), Calcum (Ca2+) and Magnesium (Mg2+). Cations that are generally considered 'unmeasured' include a few normally occurring serum proteins, and some pathological proteins (e.g., paraproteins found in multiple myeloma). Likewise, commonly 'measured' anions include chloride (Cl−), bicarbonate (HCO3−) and phosphate (H2PO4−), while commonly 'unmeasured' anions include sulfates and a number of serum proteins.
By definition, only Na+, Cl− and HCO3− (+/- K) are used when calculating the anion gap.
In normal health there are more measurable cations compared to measurable anions in the serum; therefore, the anion gap is usually positive. Because we know that plasma is electro-neutral we can conclude that the anion gap calculation represents the concentration of unmeasured anions. The anion gap varies in response to changes in the concentrations of the above-mentioned serum components that contribute to the acid-base balance. Calculating the anion gap is clinically useful, as it helps in the differential diagnosis of a number of disease states.
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