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