Arterial Blood Gases: 5 Pre-Analytical Errors You May Be Making and How to Fix Them.

Picture of arterial blood gas
Warning: Drawing your own ABG is DEFINITELY a pre analytical error!

As a Respiratory Therapist, one of the skills you will be performing most is Arterial Blood Gas (ABG) sampling and analyses.  The results of an ABG can serve as one of the most accurate ways to assess a patient’s clinical condition, and often times will be the deciding factor between whether a patient has to endure the trauma of intubation and mechanical ventilation, or, be managed less aggressively, by applying non-invasive measures.

It is absolutely imperative that Respiratory Therapists (and even RT students) make every effort to avoid the most common errors with this skill; pre-analytical errors.

What are Pre-Analytical Errors?

The term is exactly as it sounds, an error that happens before the blood sample has been introduced into the blood gas analyzer.  These errors are not about the misinterpretation of the results; they are about mistakes made that cause the results to be erroneous.

So, what sort of things are we talking about? And, how can we avoid them?

PROBLEM:  Your patient is afraid that the arterial puncture will be painful. They become anxious, and subsequently, hyperventilate.  Hyperventilation can cause a reduced pCO2 upon analysis

SOLUTION:  A respiratory therapist should use a reassuring tone when preparing for the blood sample. If necessary, an application of a topical numbing agent may help to reduce pain at the puncture site.

PROBLEM:  After drawing a sample, you fail to ensure that any air bubbles trapped inside the syringe have been expelled.  ABGs need to be collected anaerobically in order to ensure that the determined pO2 is interpreted accurately. Air trapped inside the blood-filled syringe can cause the pO2 to either increase if the patient’s actual pO2 is similar to room air, (~150mmHg), or decrease if the patient was receiving supplemental O2. 

SOLUTION:  Always ensure that once you have removed the needle from the Luer-lock on your syringe, expel all the air into a gauze pad before capping it. Pushing the barrel toward the cap will help ensure that when the blood is aspirated into the analyzer, the sample will remain anaerobic.

PROBLEM:  Using an ABG kit with dried heparin, you draw a sample and fail to mix the whole blood sample before running it through the analyzer.  Delayed mixing causes the blood to form clots, blocking the analyzer and causing the sample to be rejected.  Not only will you often find yourself having to re-draw the sample, but, the patient may experience a delay in treatment due to this mistake.

SOLUTION:  Always ensure that you have gently but thoroughly mixed your sample by inverting the syringe and rolling it through the palms of your hands. This will help to evenly distribute the heparin throughout the sample causing effective anticoagulation.

PROBLEM:  When using an  ABG syringe filled with liquid heparin, you draw the sample before expelling all of the heparin from the syringe or when drawing from an A-line you fail to ensure that whole blood is seen in your waste syringe before attaching the sample syringe.  Liquid heparin left inside the syringe will dilute the sample and cause the pCO2 to have a false low reading.

SOLUTION:  Expelling all of the heparin from the syringe before drawing the sample will prevent the over dilution of the sample.  If drawing from an A-line, fill the waste syringe until all flush solution has been removed from the catheter and whole blood appears in the syringe.

PROBLEM:  You have drawn a sample but cannot, for whatever reason, get the sample analyzed in less than 30 minutes.  A time delay between collection and analyses will cause a decrease in pO2 and an increase in pCO2 as the blood continues to metabolize even after it is drawn. 

SOLUTION:  Make every effort to limit the time between collection and analyses to less than 15 minutes. If a sample must be kept for longer than 30 minutes, the current clinical guidelines state they should be drawn and chilled in an ice slurry to slow metabolism.

ABGs are often ordered when a patient is experiencing a crisis.  Pre-analytical errors can lead to inappropriate medical treatments from the analysis of an unacceptable specimen or from incorrect reporting of results. 

As Respiratory Therapists, we must treat this responsibility with the utmost care; ensuring drawn blood samples are analyzed and interpreted correctly.

About Anne Wandycz BS, RRT-NPS-ACCS 7 Articles
Anne is a staff Respiratory Therapist at Children's Specialized Hospital, Toms River, NJ as well as an adjunct instructor in the Respiratory Care program at Brookdale Community College, Lincroft, NJ.