Acid Base Disorders

Arterial blood gas analysis is used to determine the adequacy of oxygenation and ventilation, assess respiratory function and determine the acid–base balance. These data provide information regarding potential primary and compensatory processes that affect the body’s acid–base buffering system.

Interpret the ABG in a step-wise manner:

  1. PaO2. Determine the adequacy of oxygenation
    • Normal range: 80–100 mmHg (10.6–13.3 kPa)
  2. Determine pH status
    • Normal pH range: 7.35–7.45 (H+ 35–45 nmol/L)
    • pH <7.35: Acidosis is an abnormal process that increases the serum hydrogen ion concentration, lowers the pH and results in acidaemia.
    • pH >7.45: Alkalosis is an abnormal process that decreases the hydrogen ion concentration and results in alkalaemia.
  3. PaCO2. Determine the respiratory component
    • Normal range: PaCO2 35–45 mmHg (4.7–6.0 kPa)
    • PaCO2 <35 mmHg (4.7 kPa): 
      • Primary respiratory alkalosis (hyperventilation) if pH >7.45 and HCO3 normal. 
      • Respiratory compensation for metabolic acidosis if pH <7.35 and HCO3 (decreased).
    • PaCO2 >45 mmHg (> 6.0 kPa): 
    • Primary respiratory acidosis (hypoventilation) if pH <7.35 and HCO3 normal.
    • Respiratory compensation for metabolic alkalosis if pH >7.45 and HCO3 increased.
    • In chronic respiratory acidosis the metabolic compensation may be acute or chronic. In acute compensation for every10 Δ pCO2 = 0.08 Δ pH and in chronic for every10 Δ pCO2 = 0.03 Δ pH
  4. HCO3 Determine the metabolic component
    • Normal HCO3 range 22–26 mmol/L
    • HCO3 <22 mmol/L: Primary metabolic acidosis if pH <7.35. Renal compensation for respiratory alkalosis if pH >7.45.
    • HCO3 >26 mmol/L: Primary metabolic alkalosis if pH >7.45. Renal compensation for respiratory acidosis if pH <7.35.
Rules and Resources

Additional definitions

Osmolar Gap

  • Use: Screening test for detecting abnormal low MW solutes (e.g. ethanol, methanol & ethylene glycol [Reference])
  • An elevated osmolar gap (>10) provides indirect evidence for the presence of an abnormal solute which is present in significant amounts [Reference]
  • Osmolar gap = Osmolality – Osmolarity
  • Osmolality (measured)
    • Units: mOsm/kg
    • Measured in laboratory and returned as the plasma osmolality
  • Osmolarity (calculated)
    • Units: mOsm/l
    • Osmolarity = (1.86 x [Na+]) + [glucose] + [urea] + 9  (using values measured in mmol/l)
    • Osmolarity = (1.86 x [Na+]) + glucose/18 + BUN/2.8 + 9 (using US units of mg/dl)
  • NOTE: even though the units of measured (mOsm/kg) and calculated (mOsm/l) are different [Reference], strictly they cannot be subtracted from one another… However, the value of the difference is clinically useful so the problem is usually overlooked!
Arterial Blood Gas Interpretation ABG

1 2 3 4 5 Rule

Simple table to calculate metabolic compensation in respiratory acidosis and alkalosis (aka the 1-2-3-4-5 rule)

  • Respiratory Acidosis: Increased PaCO2. For every 10mmHg rise should have INCREASE in HCO3 by 1 (acute) or 4 (chronic) as compensation
  • Respiratory Alkalosis: Decreased PaCO2. For every 10mmHg rise should have DECREASE in HCO3 by 2 (acute) or 5 (chronic) as compensation,
Simple calculation to predict changes in HCO3– from PaCO2
Simple calculation to predict changes in HCO3 from PaCO2

Critical Care


Associate Professor Curtin Medical School, Curtin University. Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |


  1. The aniongap is usually calculated without potassium en that value is used to calculate the delta ratio. What if the potassium is high? Is that a reason to include it, which will give a change in the delta-ratio compared to not using the potassium?
    This question was raised when interpreting the following ABG:
    pH 6.98; pCO2 14 mmHg (1.9 kPa); HCO3 3 mmol/l; Na 138 mmol/l; K 7.0 mmol/l; Cl 114 mmol/l

    With kind regards,

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