Pneumatic tube transport of samples for blood gas analysis

Many hospitals have a pneumatic tube system (PTS) that is routinely used to transport blood samples rapidly to the central laboratory, but the notion that such systems can be used to transport samples destined for blood gas analysis is challenged by the results of a recent study. 

This prospective study, focusing on 50 paired arterial blood samples collected from critically ill patients being cared for in intensive care, was conducted at a large (2,200 beds) tertiary care hospital in Vellore, India. The paired samples were collected simultaneously, and after ensuring elimination of air bubbles, one of the paired samples (the reference sample) was transported by courier to the laboratory and the other sent via PTS. 

The mean time from sampling to analysis was almost identical for the two modes of transport. There was excellent agreement between reference and PTS samples for pH (mean difference 0.001 pH units; 95 % limits of agreement –0.028 to +0.03), pCO₂ (mean difference 0.23 mmHg; 95 % limits of agreement –3.37 to +3.883), and bicarbonate (mean difference 0.24 mmol/L; 95 % limits of agreement –1.86 to +2.34). 

However, that was not the case for pO₂ (mean difference 0.9 mmHg; 95 % limits of agreement –40.83 to +39.02). Analysis of pO₂ data revealed that for samples with pO₂ <160 mmHg, PTS transport was associated with erroneously high pO₂, and for samples with pO₂ >160 mmHg, PTS transport was associated with erroneously low pO₂. 

Since pO₂ of ambient air is ~160 mmHg, results of the study suggest that there is clinically significant exchange of oxygen between air and blood during transit via PTS. Following this study, the proposal to use PTS for transport of blood gas samples at Vellore hospital was abandoned in the light of potential for aberrant pO₂ results; instead, a blood gas analyzer was installed in the intensive care unit. 

In discussion of their study the authors review a number of previous studies that have examined the validity of using PTS for transport of blood gas specimens as well as other issues relating to transport of blood gas samples.