Blood gases - arterial blood

Last updated: Thursday, 08, April, 2004

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1 mL arterial blood collected anaerobically in a heparinised syringe and transported rapidly to the laboratory in a capped syringe with the needle removed.

An appropriately collected 'arterialised' capillary blood specimen may have values close to those of arterial blood.

Fractional concentration of inspired O [FiO2] should be specified on request form.


Selective electrodes, chemiluminescence.

Reference Interval
pO2: 11.0-13.5 kPa (80-100 mm Hg) (varies with age)
pCO2: 4.6-6.0 kPa (35-45 mm Hg)
pH: 7.36-7.44 (36-44 nmol/L)
Base excess: (-3) to (+3) mmol/L
Alveolar-arterial pO2 difference: <3.3 kPa (<25 mmHg) if FiO2  = 0.21 (that is, room air).

Assessment of cardiopulmonary function, acid-base balance.


FiO2 and patient’s temperature must be known.

Decreased pO2 is seen with hypo­ventilation, ventilation/perfusion mismatch, alveolar-capillary block and right to left shunts.

Increased pO2 may be seen with hyperventilation or oxygen therapy.

Decreased pCO2 (respiratory alkalosis) is usually a compensatory phenomenon in metabolic acidosis, but may be a primary abnormality; in both situations it is due to hyperventilation.

Increased pCO2 (respiratory acidosis) occurs in respiratory failure, but is also seen as a compensatory phenomenon, caused by hypoventilation, in metabolic alkalosis.

Decreased pH indicates a net acidaemia and increased pH indicates a net alkalaemia. The acid-base balance component (viz metabolic or respiratory) that is in the same direction as the pH is the primary abnormality in acid-base imbalance.

Base excess is decreased in metabolic acidosis and compensated respiratory alkalosis.

It is increased in metabolic alkalosis or compensated respiratory acidosis.

Alveolar-arterial pO2 difference is elevated in all causes of hypoxia except hypoventilation. The reference interval is defined only for room air.


Walmsley RN et al. Cases in Chemical Pathology. 2nd ed. PG Publishing 1988.

Breen PH. Anesthesiol Clin North America 2001; 19(4): 885-906.