Anion Gap
AG is used to confirm that the acidosis is actually ‘metabolic’ if AG is high; and then analyse likely cause by measuring the unmeasured anions in the plasma.
Anion gap = [Na+ + K+] – [Cl– + HCO3–]
Cations (Na+ and K+) minus Anions (Cl– and HCO3–)
Normal range is 6 to 16 mmol/L
Acidosis is certainly ‘metabolic’ if the AG is greater than 30 mmol/L & mostly metabolic of AG >20
Causes:
HAGMA = High Anion Gap Metabolic Acidosis is due to accumulation of organic acids in blood
– Diabetic Ketoacidosis
– Lactic Acidosis
– Alcohol
– Drugs- Aspirin, Paracetamol, Metformin, other toxins
– Renal Failure
NAGMA = Normal Anion Gap Metabolic Acidosis is usually due to loss of HCO3–
– Diarrhoea
– Renal Tubular Acidosis
– Excess Chloride given during fluid boluses
– Addison’s
– Acetazolamide
Note: Albumin is a major unmeasured anion, and hypoalbuminemia can falsely lower Anion Gap