Guidelines for Blood Gas Analysis

Indications for blood gas analysis

  • All critically ill patients.
  • Unexpected or inappropriate hypoxaemia (SpO2 <94% in patients breathing room air or oxygen) or any patient requiring oxygen to achieve the above target range. Allowance should be made for transient dips in saturation to 90% or less in normal subjects during sleep.
  • Deteriorating oxygen saturation or increasing breathlessness in a patient with previously stable hypoxaemia (e.g. severe chronic obstructive pulmonary disease, COPD).
  • Any previously stable patient who deteriorates and requires a significantly increased FiO2 to maintain a constant oxygen saturation.
  • Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, deteriorating oxygen saturation, drowsiness or other symptoms of carbon dioxide retention e.g. COPD or neuromuscular disorders.
  • Acutely breathless or with poor peripheral circulation in whom a reliable oximetry signal cannot be obtained.
  • Severe metabolic disturbance e.g. diabetic ketoacidosis (DKA) or renal failure, hypothermia (temperature <32°C), severe sepsis, shock or altered conscious level.
  • Any other evidence from the patient’s medical condition that would indicate that blood gas results would be useful in the patient’s management (e.g. an unexpected change in the NEWS score or an unexpected fall in oxygen saturation of 3% or more, even if within the target range).
  • Smoke inhalation / carbon monoxide poisoning / cyanide poisoning.
  • In patients who may require multiple arterial blood gas samples, admission to a HDU environment and siting of an arterial line should be considered.
  • Patients who have markedly deranged arterial blood gas parameters are likely to be critically unwell and escalation to high dependency or intensive care should be considered.

Local anaesthetic skin infiltration should be considered prior to obtaining an arterial blood gas sample. 

Interpretation of blood gases

See figure 1 for blood gas analysis algorithm and figure 2 for further information, including possible causes.

Venous blood gas analysis

A venous blood gas can be used in situations where monitoring of pCO2 and H+ is desirable without multiple arterial sampling. The following corrections should be taken into account when interpreting the results.

N.B. There is no way to correlate pO2 between venous blood gases and arterial blood gases.

Peripheral venous gases

Obtained by peripheral venipuncture. The tourniquet should be removed 1 minute prior to sampling to ensure local ischaemia does not adversely affect the results.

  • H+ concentration usually 2-4nmol/L higher compared with arterial samples. pH will be 0.02-0.04 lower than the arterial sample. 
  • pCO2 is 0.4-1.1kPa (3-8mmHg) higher.

Central venous (Sv) gases

Aspirated from a central line.

  • H+ concentration usually 3-5nmol/L higher compared with arterial samples. pH will be 0.03-0.05 lower than the arterial sample.
  • pCO2 is 0.5-0.6kPa (4-5mmHg) higher.


Content last updated: May 2022

Page last updated: June 2022