Sample Stability and PO2–A Learning Opportunity

One of the interesting things about working in the field of laboratory medicine is that there are always opportunities for learning new things. Almost every call I get from my colleagues outside the lab allows me and the lab team these opportunities. And sometimes we are reminded of the reason we do the things we do, basically re-learning them.

Case in point: An ICU physician contacted the lab, understandably concerned. He had been monitoring the pO2 in a patient using an I-Stat point of care analyzer. Values had been in the range of 50-70 mmHg, and he had been adjusting ventilation on the basis of those results. A blood gas sample was sent to the main lab, analyzed on an ABL analyzer and gave a result of 165 mmHg, repeated shortly thereafter on a new sample with a 169 mmHg. Understandably, the physician wanted to know which analyzer was wrong and how he should be adjusting his patient’s ventilation.

We quickly did an investigation and determined an interesting fact that we hadn’t paid much attention to previously. A blood gas sample that is sent through the tube system that has any amount of air in the sample, will give falsely elevated pO2 result. We investigated this by collecting blood gas samples, running them on both the I-Stat and the ABL, and then sending them through the tube system and rerunning them on both instruments after tubing. The pO2 values matched on both instruments, both before and after tubing. But interestingly, if there was any air in the collection device when the device was sent through the tube system, the pO2 after tubing still matched on the two instruments, but the values were more than double the original values. If no air was present, there was very little change before and after tubing. We tested this by expressing all air from one set of samples before tubing and leaving air in the syringe on the other set.

The collection process for blood gas samples in our institution has always specified that the collector should express any air in the sample before sending the sample to the lab through the tube system, and after this incident the reason for that step became clear. However, the staff collecting blood gases on the floors needs to be periodically retrained in the collection, and the lab staff needs to be reminded that air in a blood gas syringe arriving through the tube station is a reason to reject the sample. We were reminded that education needs to be a continuous process. We also learned that when we discover the reason for a process, it’s a good idea to document that reason in order to both understand the need and to help motivate people to follow it.

-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the Chemistry and Metabolic Disease Laboratories at Children’s Medical Center in Dallas, TX and a Professor of Pathology at University of Texas Southwestern Medical Center in Dallas.

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