What is the comparative accuracy of Capillary Blood Gas (CBG) to Arterial Blood Gas (ABG) and Venous Blood Gas (VbG)?

From the Guidelines

Capillary blood gas (CBG) sampling is less accurate than arterial blood gas (ABG) sampling, but can be used in place of ABG for re-measuring PaCO2 and pH at different oxygen flow rates during LTOT assessment, as stated in the British Thoracic Society guidelines 1. When comparing CBG to ABG and venous blood gas (VbG), it's essential to consider the context and purpose of each measurement.

• CBGs tend to provide reasonably accurate pH and bicarbonate values compared to ABGs, with differences typically within clinically acceptable ranges.
• However, CBG measurements of PO2 are significantly less reliable, often underestimating arterial oxygen levels, making them unsuitable for precise oxygenation assessment.
• PCO2 values from CBGs may differ from ABGs by approximately 3-6 mmHg.
• VBGs typically have lower pH, higher PCO2, and much lower PO2 values than arterial samples. The British Thoracic Society guidelines recommend using ABG sampling for initial assessment and confirmation of the need for LTOT, but allow for CBG sampling during oxygen titration 1.
• The guidelines suggest that CBG can be used to re-measure PaCO2 and pH at different oxygen flow rates, but not for initial assessment or confirmation of LTOT need.
• This approach prioritizes the accuracy of ABG sampling while acknowledging the potential benefits of CBG sampling in specific contexts, such as oxygen titration during LTOT assessment. In clinical practice, the choice between CBG, ABG, and VbG sampling should be guided by the individual patient's needs and the specific clinical context, with a focus on minimizing morbidity, mortality, and improving quality of life.
• CBGs may be most appropriate for patients requiring serial measurements or when arterial sampling is difficult, but should not replace ABGs when precise oxygenation assessment is critical.
• The accuracy of CBGs depends heavily on proper technique, adequate perfusion, and patient warming to arterialize the sample, as emphasized in the guidelines 1.

From the Research

Comparison of CBG, ABG, and VBG in Terms of Accuracy

• The accuracy of capillary blood gas (CBG), arterial blood gas (ABG), and venous blood gas (VBG) sampling methods has been compared in various studies 2, 3, 4, 5, 6.
• ABG analysis is considered the gold standard for evaluating the partial pressures of gas in blood and acid-base content, providing a clear understanding of respiratory, circulatory, and metabolic disorders 2.
• VBG sampling has been proposed as a potentially more accessible alternative to ABG sampling, with studies suggesting that pH and Pco2 obtained via peripheral VBG correlate well with ABG measurement 3.
• However, the value of using central VBG to guide clinical decisions or as a surrogate for ABG is unclear, with some studies showing a mean difference between arterial and central venous pH and Pco2 of 0.03 units and 4-6.5 mm Hg, respectively 3.
• CBG sampling has been compared to ABG and VBG sampling in a study, showing close agreement between ABG and mathematically arterialised venous blood gas sampling (v-TAC), but not between ABG and CBG or VBG 4.
• The study found that v-TAC sampling had the highest first-time success rate and was less painful than arterial sampling, suggesting that it could replace routine ABG testing to recognise and monitor patients with hypercapnic respiratory failure 4.
• Another study found that VBG pH and HCO3 were useful in assessing patients with acute exacerbations of chronic obstructive pulmonary disease, but VBG pCO2 was considered too unpredictable 6.

Key Findings

• ABG analysis is the gold standard for evaluating blood gas and acid-base content 2.
• VBG sampling may be a useful alternative to ABG sampling in certain situations, but its value is unclear 3.
• CBG sampling may not be as accurate as ABG or v-TAC sampling 4.
• v-TAC sampling may be a useful tool for monitoring patients with hypercapnic respiratory failure 4.
• VBG pH and HCO3 may be useful in assessing patients with acute exacerbations of chronic obstructive pulmonary disease, but further study is needed on the possible role of VBG in the management of such patients 6.