What is the difference between an Arterial Blood Gas (ABG) sample and a Venous Blood Gas (VBG) sample?

Differentiating Between ABG and VBG Samples

Arterial blood gas (ABG) samples arterial blood and provides the gold standard for assessing oxygenation (PaO2), ventilation (PaCO2), and acid-base status (pH), while venous blood gas (VBG) samples venous blood and can reliably assess pH and CO2 but cannot accurately measure oxygenation. 1

Key Distinguishing Features

Sample Source and Collection Site

• ABG is obtained from arterial sources, most commonly the radial artery after performing an Allen's test to confirm dual blood supply to the hand 2, 3, 1
• VBG is obtained from venous sources, typically the cubital vein or dorsal hand veins 4
• Arterial samples are most similar to laboratory plasma or blood gas analyzer values in paired samples 2
• When using VBG, care must be taken to avoid contamination from IV fluid infusing through multilumen catheters 2, 1

Clinical Parameters and Accuracy

pH Assessment:

• ABG and VBG show very high agreement for pH, with mean difference of only 0.03-0.05 units 5, 6, 4
• The correlation coefficient between arterial and venous pH is 0.87-0.94 6, 7
• Conversion formula: arterial pH = venous pH + 0.05 units 5

CO2 Measurement:

• ABG and VBG correlate well for PCO2, with mean difference of 4-6.5 mm Hg in stable patients 5, 6
• The correlation coefficient for PCO2 is 0.83-0.93 6, 7
• Conversion formula: arterial PCO2 = venous PCO2 - 5 mm Hg 5
• VBG has 100% sensitivity and 93.8% specificity for detecting hypercapnia (PaCO2 >6.0 kPa) 8

Oxygenation Assessment:

• This is the critical differentiator: VBG cannot accurately measure arterial oxygenation 1
• Mean difference for PO2 between VBG and ABG is 53.6 mm Hg, with poor correlation (r=0.287-0.31) 6, 7
• ABG remains essential when precise PaO2 measurement is required 1

Clinical Decision Algorithm

When ABG is Mandatory:

• All critically ill patients requiring assessment of oxygenation 3, 1
• Patients in shock, on vasopressor therapy, or with severe peripheral edema 2, 3
• Post-cardiac arrest patients to guide oxygen therapy 3, 9
• Patients on ECMO (samples from right radial arterial line to represent cerebral perfusion) 3, 9
• Cardiogenic shock patients requiring precise acid-base assessment 1, 9
• When calculating A-a gradient 3

When VBG May Be Acceptable:

• Stable patients without severe acid-base disturbances for pH and PCO2 assessment 5
• Screening for arterial hypercapnia 5
• When combined with pulse oximetry (SpO2) for undifferentiated critically ill patients in ED/ICU settings 6
• Hypotensive patients when mathematical conversion formulas are applied 4

Critical Pitfalls to Avoid

Common Misconceptions:

• A normal SpO2 does not rule out significant acid-base disturbances or hypercapnia - ABG is still necessary 3, 1
• Pulse oximetry will appear normal in patients with normal PO2 but abnormal pH or PCO2 1, 9
• In patients with circulatory failure, the difference between central venous and arterial pH/PCO2 is 4-fold greater than in stable patients 5

Technical Considerations:

• Finger-stick capillary glucose levels provide significantly different results compared with arterial or venous specimens in patients with low perfusion, hypotension, edema, or vasopressor infusion 2
• Local anesthesia should be used for all ABG specimens except in emergencies 3, 1, 9
• Arterial samples should be obtained from existing arterial lines when available to minimize complications 3

Practical Sampling Hierarchy

For critically ill patients, establish this sampling priority: 2

1. Arterial sampling (first choice for all parameters)
2. Venous sampling (acceptable for pH/PCO2 in stable patients, with proper technique to avoid IV contamination)
3. Finger-stick capillary sampling (site of last resort, avoid completely in patients on vasopressors or with hypoperfusion)