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The anion gap is the quantity difference between cations (positively charged ions) and anions (negatively charged ions) in serum, plasma, or urine. The magnitude of this difference (i.e., "gap") in the serum is calculated to identify metabolic acidosis. If the gap is greater than normal, then high anion gap metabolic acidosis is diagnosed.
The anion gap (AG) without potassium is calculated first and if a metabolic acidosis is present, results in either a high anion gap metabolic acidosis (HAGMA) or a normal anion gap acidosis (NAGMA). A low anion gap is usually an oddity of measurement, rather than a clinical concern.
Urine NH 4 + is difficult to measure directly, but its excretion is usually accompanied by the anion chloride. A negative urine anion gap can be used as evidence of increased NH 4 + excretion. In a metabolic acidosis without a serum anion gap: A positive urine anion gap suggests a low urinary NH 4 + (e.g. renal tubular acidosis).
(The serum potassium concentration may be added to the calculation, but this merely changes the normal reference range for what is considered a normal anion gap) Because the concentration of serum sodium is greater than the combined concentrations of chloride and bicarbonate an 'anion gap' is noted.
However, the solutes contributing to this total differ. Serum is mostly sodium and potassium salts (as reflected in the formulas for serum osmol gap and anion gap), while the digestive tract contains significant amounts of other compounds. Stool osmotic gap is a measure of the concentration of those other compounds. [citation needed]
Hyperchloremic acidosis is a form of metabolic acidosis associated with a normal anion gap, a decrease in plasma bicarbonate concentration, and an increase in plasma chloride concentration [1] (see anion gap for a fuller explanation).
The serum anion gap is useful for determining whether a base deficit is caused by addition of acid or loss of bicarbonate. Base deficit with elevated anion gap indicates addition of acid (e.g., ketoacidosis). Base deficit with normal anion gap indicates loss of bicarbonate (e.g., diarrhea).
Several types of metabolic acidosis occur, grouped by their influence on the anion gap. The anion gap can be increased due to relatively low levels of cations other than sodium and potassium (e.g. calcium or magnesium). An anion gap is usually considered to be high if it is over 12 mEq/L. High anion gap metabolic acidosis is typically caused by ...