Most ABG machines are so sophisticated that they rarely make errors. Most studies seem to confirm this, noting that as many as 68 percent of ABG analytical errors are handling errors that occur between the time of the draw and insertion of the syringe in the ABG machine, according to John J. Ancy in his RTmagazine.com article, "Blood Gases and Preanalytical Error Prevention."
The following are possible preanalytical errors:
1. Exposure to air:
- Problem: Whole blood continues to metabolize after the draw, and for this reason it's important to have proper handling of the sample. There is a bubble in the syringe.
- Error: Trapped air in the syringe causes the PO2 to move toward 150, which is the PO2 of room air. So if the actual PO2 is less than 150, the PO2 reading may be inaccurately high as it moves up towards 150. If the actual PO2 is greater than 150, the PO2 reading may be inaccurately low as it moves down toward 150. CO2 become slightly lower with a slight rise in pH.
- Solution: Tap bubbles from syringe and aspirate air into a filter immediately after the draw.
2. Improper mixing of heparin:
- Problem: If dry heparin is not mixed with the blood clots may form and readings may not be accurate
- Error: Clotting cannot be reversed. Clots may cause to machine to break down.
- Solution: A small amount of dry heparin will prevent clots. Make sure to mix the syringe, roll it between your fingers, for about one minute, and then expelling a few drops into a gauze pad, both prior to inserting the syringe into machine. The flea in capillary blood gases should be used to mix the sample for five seconds.
3. Ice storage
- Problem: Past policies recommended storing the post draw ABG in a slurry of ice, although new recommendations frown upon this. It's difficult to get outdated hospital policies changed.
- Error: It reduces metabolism of the blood in the syringe, but the new plastic syringes are permeable to outside oxygen molecules. Cooling increases hemoglobin's affinity for oxygen, and this may attract oxygen molecules from ambient air to these hemoglobin molecules. This may artificially inflate PO2.
- Solution: Do not use an ice slurry. Instead, assure that the blood is inserted into the analyser within 30 minutes of the draw. New recommendations suggest only placing the ABG on ice if the time from draw to ABG analyzer is longer than 30 minutes. I wrote more about this here.
4. Art line draw:
- Problem: Heparin is needed to keep blood from clotting in the art line system
- Error: Heparin in the syringe may cause inaccurate results
- Solution: You must waste two times the dead space in the system. How do you know the dead space volume. Ancy explains: "If vascular line dead space is unknown, turn the stopcock to the sampling port and withdraw flush solution until blood appears in the hub of waster syringe. The volume in the syringe at that point will be equal to the dead space, double that volume for the waste draw."
5. Changes:
- Problem: Changes in patient settings may affect ABG results: Peep changes, oxygen changes, suctioning, ventilator settings, etc.
- Error: ABG results may be artificially high or low
- Solution: Most recommendations suggest waiting 20-30 minutes after changing oxygen, ventilator settings, or PEEP/CPAP.
6. Temperature Correction:
- Problem: Some experts recommend if a patient has a fever the temperature on the analyzer should be adjusted to match the patient temperature. It's difficult to change past hospital policies, and it's difficult to get doctors to understand that methods of interpreting corrected and uncorrected temperature readings may be different. For this reason, most medical experts recommend NOT making any corrections in temperature. In other words, there are no reference ranges for other temperatures other than room temperature. I wrote about this here.
- Error: Temperature can inadvertently affect ABG results by speeding up metabolism.
- Solution: At present, the recommendation is not to make any temperature correction. Ideally, there should be reference ranges for ABGs at all temperatures. Temperatures should be corrected only at the recommendation of the physician, and any changes in temperature should be reported in the comments. Interpretation of the results is to be done by the physician who requests them.
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