The importance of correct blood sampling for blood gas analysis

The significance of preanalytical technique for accurate arterial blood gas analysis is the focus of a recently published study. The authors of this study sought to quantitate the analytical error associated with collection of venous rather than arterial blood, delay in arterial blood analysis and contamination of arterial blood with air. 

Arterial and venous blood was collected from 30 patients with acute exacerbation of chronic obstructive pulmonary disease and 30 "healthy" volunteers. Venous samples were analyzed immediately. Arterial blood specimens were separated into " air-contaminated " and "non-air-contaminated" samples and analyzed immediately they were sampled and then at 30, 60, 90 and 180 minutes after collection. 

Results confirm a clinically significant difference in venous blood pH compared with arterial blood pH (mean venous blood pH 7.371, mean arterial blood pH 7.407, p <0.0001). The pH of arterial blood was shown to decline by a statistically significant amount within 30 minutes of collection and by a clinically significant amount within 73 minutes of collection. 

Even when measured immediately, air contamination was associated with clinically significant increase in arterial pO2. The authors conclude that venous blood pH cannot be used to accurately predict arterial blood pH, delay in analysis beyond 30 minutes leads to inaccurate arterial blood pH and samples contaminated with air cannot be used to assess arterial pO2.