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ABG Interpretation

Assessing the pH

March 4th, 2023
On this page:AcidaemiaAlkalaemia


The serum pH is a measure of the acidity of alkalinity of the blood, based on an inverse log of the hydrogen ion concentration. This can be used in conjunction with the HCO₃ and PaCO₂ to determine whether there is an acid-base disorder present.
    • Normal Range

    • 7.35 - 7.45


A pH <7.35 is indicative of acidaemia; this may be due to a primary metabolic or respiratory acidosis.
  • Causes of Acidaemia

  • Metabolic acidosis - acid ingestion, lactic acidosis, ketoacidosis, renal failure, renal tubular acidosis, diarrhoea
  • Respiratory acidosis - hypoventilation or dead space ventilation
  • Interpretation

  • Reduced HCO₃ ± reduced PaCO₂ - suggests metabolic acidosis
  • Elevated PaCO₂ ± elevated HCO₃  - suggests respiratory acidosis


A pH >7.45 is indicative of alkalaemia; this may be due to a primary metabolic or respiratory alkalosis.
  • Causes of Alkalaemia

  • Metabolic alkalosis - bicarbonate excess, GI acid loss, renal acid loss, compartmental shift
  • Respiratory alkalosis - hyperventilation
  • Interpretation

  • Elevated HCO₃ ± elevated PaCO₂ - suggests metabolic alkalosis
  • Reduced PaCO₂ ± reduced HCO₃ - suggests respiratory alkalosis
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