When to Obtain Arterial vs Venous Blood Gas
Obtain an arterial blood gas (ABG) for all critically ill patients and those with shock or hypotension (systolic BP <90 mmHg), as arterial sampling is mandatory in these situations to accurately assess oxygenation and guide life-saving interventions. 1
Critical Situations Requiring ABG
Always obtain ABG in:
- Critically ill patients - This is non-negotiable for accurate assessment of oxygenation status 1
- Shock or hypotension (systolic BP <90 mmHg) - Peripheral perfusion may be compromised, making venous samples unreliable 1
- When accurate PO2 measurement is essential - VBG cannot assess oxygenation; arterial PO2 is typically 36.9 mmHg higher than venous with significant variability 2
When VBG May Be Acceptable
For most non-critically ill patients requiring blood gas sampling, venous blood gas (VBG) or arterialised earlobe samples can provide accurate pH and PCO2 measurements. 1
VBG is reasonable for:
- Screening for metabolic acidosis - High sensitivity (80.6%) and specificity (89.5%) for detecting metabolic acidosis 3
- Diabetic ketoacidosis diagnosis - VBG is sufficient for making this diagnosis 4
- Lactate measurement - Venous lactate shows 96% agreement with arterial values within clinically acceptable limits 5
- pH assessment - Arterial pH is typically only 0.03 higher than venous pH (95% CI: 0.029-0.038), which is clinically acceptable 2
Important Limitations of VBG:
- Cannot assess oxygenation - Use pulse oximetry as a non-invasive alternative for oxygen monitoring 4
- PCO2 less reliable - Venous PCO2 underestimates arterial by 3.6 mmHg with wide limits of agreement (-15 to +8.1 mmHg) 3, 2
- Cannot calculate anion gap or ventilatory compensation reliably 4
Clinical Algorithm for Decision-Making
Step 1: Assess patient acuity
- Critical illness, shock, or hypotension → ABG mandatory 1
Step 2: Determine what information is needed
- Need accurate oxygenation status → ABG required 1
- Screening for acid-base disturbance only → VBG acceptable 4, 3
- Assessing metabolic acidosis or lactate → VBG acceptable 5, 3
Step 3: Consider supplemental oxygen status
- Patient on supplemental oxygen with normal SpO2 → Blood gas still needed (pulse oximetry can miss abnormal pH/PCO2) 1
Key Clinical Scenarios Requiring Blood Gas Assessment
The BTS guidelines specify blood gases should be checked in: 1
- All critically ill patients (use ABG)
- Unexpected fall in SpO2 below 94% in patients on air or oxygen
- Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in chronic hypoxemia (e.g., severe COPD)
- Risk factors for hypercapnic respiratory failure with acute breathlessness, drowsiness, or CO2 retention features
- Suspected metabolic conditions like diabetic ketoacidosis or renal failure-related metabolic acidosis
- Patients requiring increased FiO2 to maintain constant oxygen saturation
Important Caveats
Normal pulse oximetry does not eliminate the need for blood gas measurement, especially in patients on supplemental oxygen, as SpO2 will be normal despite abnormal pH, PCO2, or anemia. 1
Use local anesthesia for all ABG specimens except in emergencies to minimize patient discomfort. 1
If using VBG for screening and there is any concern about accuracy, obtain a confirmatory arterial sample, particularly if clinical decision-making depends on precise values. 1