Is it acceptable to use venous rather than arterial blood for blood gas testing of trauma patients?

Although arterial blood is universally agreed to be the “gold standard” sample for blood gas analysis, it is, compared with other blood samples (venous/capillary), difficult to obtain, and associated with higher risk and more pain for the patient. It would be logistically more convenient for clinical staff, as well as beneficial for the patient, if venous rather than arterial blood could be used for blood gas analysis.

These considerations have driven an interest in study aimed at determining if venous blood gas results reflect those obtained from arterial blood with clinically acceptable level of agreement. An ever-growing number of such studies focusing on different patient groups have been published over the past 15-20 years with conflicting results. There seems, for example, to be an established consensus that venous blood is an acceptable alternative sample for assessment of acid-base status among patients with diabetic ketoacidosis, but venous blood is not necessarily acceptable for assessment of pCO₂ in patients with respiratory disease. All agree that venous blood is unacceptable if the purpose of blood gas testing is to assess patient oxygenation status (pO₂ and sO₂).

The study highlighted here, which is the latest to examine the clinical acceptability of using venous rather than arterial blood for blood gases, focuses on trauma patients. The study was limited to arterial vs. venous comparison of just three blood gas parameters: pH, base deficit (BD) and lactate.

The study population comprised 102 adult trauma patients admitted to a tertiary trauma center in Singapore. Recruitment to the study depended on emergency physician assessment that blood gas analysis was clinically warranted.

Arterial and venous blood was sampled from each study patient. The time between collection of the two samples was <10 minutes. All samples were analyzed within 5 minutes of collection, and pH, BD and lactate results from venous and arterial samples recorded for comparative analysis.

The authors of the study had, prior to data collection, determined the following clinically acceptable limits of agreement between venous and arterial samples:

• pH of venous blood should be within ±0.05 pH units of arterial pH
• BD of venous blood should be within ±2 mmol/L of arterial BD, and
• Lactate of venous blood should be within ±1.5 mmol/L of arterial lactate

By these predefined limits, only 72.6 % of venous pH measurements and 76.5 % of venous BD measurements were in agreement with arterial values.
However, 96.5 % of venous lactate measurements were within the predefined limits of agreement with arterial lactate values.

The authors conclude that arterial blood sampling is necessary for evaluation of pH and BD in trauma patients; but both venous and arterial blood samples are acceptable for evaluation of lactate in this patient group.

In discussion of their study, the authors briefly review the clinical utility of measuring pH, BD and lactate in trauma patients, and outline the results of other studies examining the validity of using venous rather than arterial blood for blood gas testing of trauma patients.