Metabolic Acidosis Evaluation


  • A metabolic acidosis is a process which, if uncorrected, would lead to an acidaemia.
  • It is usually associated with a low bicarbonate concentration (or total CO2), but an acidosis may be masked by a co-existing metabolic alkalosis.


  • accumulation of acids (measured, i.e. chloride hyperchloraemic metabolic acidosis] or unmeasured [increased anion gap metabolic acidosis])
  • renal or gastrointestinal loss of bicarbonate (with absorption of chloride, resulting in hyperchloraemic metabolic acidosis).


Anion gap = (Na + K) – (Cl + HCO3) simplified as (Na) – (Cl + HCO3)

  • is usually determined primarily by negatively charged plasma proteins
  • range = 10 to 16 mmol/L (8 to 12 mmol/L if K not included)
  • AG decreases by about 2.5 mmol/L for every decrease in albumin by 10 g/L
  • increased anion gap -> fall in unmeasured cations (Ca, Mg) or increase in unmeasured anions (lactate, ketoacids, formate (methanol), glycolate and oxlate (ethylene glycol))

High anion gap (HAGMA)

  • Lactate
  • Toxins – methanol, metformin, phenformin, paraldehyde, propylene glycol, pryroglutamic acidosis, iron, isoniazid, ethanol, ethylene glycol, salcylates, solvents
  • Ketones
  • Renal

Normal anion gap (NAGMA)

  • Chloride
  • Acetazolamide and Addisons
  • GI causes – diarrhoea, vomiting, fistulas (pancreatic, ureterostomies, small bowel, ileostomies)
  • Extras – RTA


  • Check delta ratio in HAGMA to determine if there is a coexistant NAGMA.
  • osmolar gap can help as a screening test for methanol or ethylene glycol intoxication once alcohol has been excluded (calculated osmolality = 2*Na + Glucose + Urea + ethanol/4.6).
  • urinary pH (inappropriately alkaline for an acidaemia) and electrolytes may facilitate eliciting the specific cause of the renal bicarbonate loss (e.g. renal tubular acidosis).

References and Links

CCC 700 6

Critical Care


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