Anion Gap
The anion gap is the difference in the measured cations and anions. It is calculated by:
( [Na+] + [K+] ) - ( [Cl-] + [HCO3-] )
A normal anion gap is <11 mEq/L.
The anion gap is often useful in determining the cause of acidosis. Acidosis may be categorised into high anion gap, normal anion gap or low anion gap.
In high anion gap conditions there is often an increase in acid e.g. ketoacids in DKA with a drop in bicarb ions (HCO3-) which are used up buffering them
In normal anion gap conditions the drop in (HCO3-) is compensated for by an increase in chloride ions and is sometimes referred to as hyperchloremic acidosis
In low anion gap conditions the cause is typically either a decrease in unmeasured anions (e.g. hypoalbuminaemia or haemodilution) or increase in unmeasured cations (e.g. multiple myeloma or hypercalcaemia/hypermagnesaemia)
( [Na+] + [K+] ) - ( [Cl-] + [HCO3-] )
A normal anion gap is <11 mEq/L.
The anion gap is often useful in determining the cause of acidosis. Acidosis may be categorised into high anion gap, normal anion gap or low anion gap.
In high anion gap conditions there is often an increase in acid e.g. ketoacids in DKA with a drop in bicarb ions (HCO3-) which are used up buffering them
In normal anion gap conditions the drop in (HCO3-) is compensated for by an increase in chloride ions and is sometimes referred to as hyperchloremic acidosis
In low anion gap conditions the cause is typically either a decrease in unmeasured anions (e.g. hypoalbuminaemia or haemodilution) or increase in unmeasured cations (e.g. multiple myeloma or hypercalcaemia/hypermagnesaemia)
Causes of acidosis by anion gap
| High anion gap | Normal anion gap | Low anion gap |
|---|---|---|
| Methanol Uremia (chronic kidney failure) Ketoacidosis Paraldehyde/phenformin Iron overload Infection Isoniazid Inborn errors of metabolism Lactic acidosis Glycols (Ethylene and Propylene) Salicylates Paracetamol overdose | GI loss of HCO3 e.g. diarrhoea, vomiting Renal loss of HCO3 e.g. renal tubular acidosis Hypoaldosteronism Addisons Ammonium chloride ingestion Acetazolamide Saline rehydration | Hypoalbuminaemia Myeloma Hypercalcaemia Hypermagnesaemia Lithium overdose Bromide overdose |