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an opening or hiatus.
anion gap the concentration of plasma anions not routinely measured by laboratory screening, accounting for the difference between the routinely measured anions and cations and equal to the plasma sodium − (chloride + bicarbonate); used in the evaluation of acid-base disorders.
auscultatory gap a period in which Korotkoff sounds disappear during auscultation of a patient' s blood pressure.
the difference between the sum of the measured cations and anions in the plasma or serum calculated as follows: ([Na+ + [K+]) - ([Cl-] + [HCO3-]) = < 20 mmol/L. Elevated values may occur in diabetic or lactic acidosis and in various types of poisoning; normal or low values occur in bicarbonate-losing metabolic acidoses.
Synonym(s): cation-anion difference
anion gapA mathematic approximation of the difference between unmeasured anions (PO4–, SO4–, proteins and organic acids) and unmeasured cations (Ca2+, Mg2+), with the former usually exceeding the latter. The anion gap is the difference between the sum of the most abundant measured serum anions (Cl– and HCO3–) and serum cations (Na+, K+). Urinary AG is calculated as (Na+) + (K+) – (Cl–); it is a crude index of the levels of urinary ammonium and used to evaluate hyperchloremic metabolic acidosis.
Serum, heparinised plasma.
Increased anion gap
Renal failure due to defective renal tubular acidification, with an increase in phosphate and sulfate; starvation-related ketoacidosis due to an accumulation of acetoacetate and beta-hydroxybutyrate, or alcohol abuse; in disorders of amino acid metabolism and hyperglycaemic nonketotic coma due to various organic acids; in lactic acidosis, overdoses or poisoning (e.g., salicylates, methanol ethylene glycol antifreeze or paraldehyde).
Decreased anion gap
Hypermagnesaemia; GI loss of bicarbonate; in nephrotic syndrome due to a loss of albumin, which is anionic at a physiologic pH; after lithium ingestion; and in myeloma and Waldenström’s macroglobinaemia, due to an increase in cationic proteins.
anion gapLab medicine A mathematic approximation of the difference between unmeasured anions–PO4–, SO4–, proteins and organic acids, and unmeasured cations-Ca2+, Mg2+, which normally exceed unmeasured cations; the AG is the difference between the sum of the most abundant measured serum anions–Cl– and HCO3– and serum cations–Na+ and K+; urinary AG is calculated as Na+ + K+ – Cl– and is a crude index of the levels of urinary ammonium and used to evaluate hyperchloremic metabolic acidosis Ref range 8-16 mEq/L; AG is ↑ in renal failure due to defective renal tubular acidification with an ↑ in phosphate and sulfate, starvation-related DKA due to an accumulation of acetoacetate and β-hydroxybutyrate or alcohol abuse, in disorders of amino acid metabolism and hyperglycemic nonketotic coma due to various organic acids, in lactic acidosis, overdose or poisoning—eg salicylates, methanol ethylene glycol antifreeze or paraldehyde; AG is ↓ in hypermagnesemia, GI loss of bicarbonate, in nephrotic syndrome due to a loss of albumin which is anionic at a physiological pH, after lithium ingestion, and multiple myeloma and Waldenström's macroglobinemia, due to an ↑ in cationic proteins
an·i·on gap(an'ī-on gap)
The arithmetic difference between the concentrations of routinely measured cations (Na+ + K+) and of routinely measured anions (Cl- + HCO3-) in plasma or serum; unmeasured anions (phosphate, sulfate, protein, other organic ions) account for the gap, which is increased in metabolic acidosis due to diabetic ketosis, renal failure, or extraneous substances (methanol, salicylate).