Master Of Medicine

Anion Gap is of 2 types, plasma Anion gap and urinary anion gap

Plasma Anion Gap

Plasma Anion Gap refers to the amount of unmeasured anions present in the plasma.

Calculated by:[ Na+]     –     [Cl-]+[HCO3-]  or

[Na+] + [K+] – [Cl-]  + [HCO3-]

Normal Anion Gap- 10-12 meq

Delta Gap= Anion gap-12

Unmeasured anions-Proteins,phosphate,Sulfate,organic anions.

Points to be noted

1. When the ion added is chloride, anion gap remains the same.
2. Eg:Adding HCl leads to decrease in HCO3-(used for buffering H+) but the decrease in HCO3- is balanced by an increase in Cl-.So it leads to normal anion gap metabolic acidosis,also called hyperchloremic metabolic acidosis.
3. When the ion added is not chloride, anion gap widens.

Urine Anion Gap

[Na+]+[K+]-[Cl-]

In normal subjects urine anion gap is usually zero or positive.

Urine anion gap is negative if there is no distal acidification of urine.

In Renal Tubular acidosis Urine anion gap is normal.

High anion gap metabolic acidosis

A high anion gap indicates acidosis. e.g. In uncontrolled diabetes, there is an increase in ketoacids (i.e. an increase in unmeasured anions) and a resulting increase in the anion gap. In these conditions, bicarbonate concentrations decrease, in response to the need to buffer the increased presence of acids (as a result of the underlying condition). The bicarbonate is replaced by the unmeasured anion resulting in a high anion gap.

• Methanol
• Uremia
• DKA/Alcoholic KA
• Paraldehyde/propylene glycol(used in diazepam injns)
• Isoniazid
• Lactic Acidosis
• Ethylene Glycol
• Rhabdo/Renal Failure
• Salicylates

Mnemonic:MUDPILERS

Normal Anion gap Metabolic acidosis

In patients with a normal anion gap the drop in HCO₃⁻ is compensated for almost completely by an increase in Cl⁻ and hence is also known as hyperchloremic acidosis.

The HCO₃⁻ lost is replaced by a chloride anion, and thus there is a normal anion gap.

• Gastrointestinal loss of HCO₃⁻ (i.e., diarrhea) (vomiting causes hypochloraemic alkalosis)
• Renal loss of HCO₃⁻ (i.e. proximal renal tubular acidosis(RTA) also known as type 2 RTA)
• Renal dysfunction (i.e. distal renal tubular acidosis also known as type 1 RTA)
• Ingestions
  • Ammonium chloride and Acetazolamide.
  • Hyperalimentation fluids (i.e. total parenteral nutrition)
• Some cases of ketoacidosis, particularly during rehydration with Na+ containing IV solutions.
• Mineralocorticoid deficiency (Addison’s disease)

FUSEDCARS (fistula (pancreatic), uretogastric conduits, saline administration, endocrine (hyperparathyroidism), diarrhea, carbonic anhydrase inhibitors (acetazolamide), ammonium chloride, renal tubular acidosis, spironolactone)

Summed up as: