Why Arterial Blood Gas Samples Require Heparin and Ice Packing
Arterial blood gas (ABG) samples are collected with heparin to prevent clotting and transported on ice to minimize metabolic activity that would alter gas measurements, ensuring accurate results that reflect the patient's true physiological status.
Heparin Use in ABG Collection
Purpose of Heparin
- Heparin serves as the only appropriate anticoagulant for ABG analysis, preventing blood clot formation that would:
- Block analyzer pathways
- Invalidate test results
- Potentially damage expensive equipment
Proper Heparin Technique
- Recommended approach for accurate results:
- Use pre-heparinized syringes (ideally with lyophilized/dried heparin)
- Maintain heparin concentration below 200 IU/mL of blood 1
- Ensure blood dilution is less than 5% when using liquid heparin
- Mix blood and anticoagulant thoroughly immediately after sampling
Potential Errors with Improper Heparin Use
Excessive liquid heparin can:
- Dilute the sample (creating a 1.25% error even with minimal residual heparin)
- Artificially decrease PCO₂, PO₂, HCO₃, and base excess values
- Produce false results suggesting metabolic acidosis with respiratory compensation 2
- Interfere with electrolyte measurements, particularly ionized calcium
Inadequate mixing with heparin leads to:
- Small clot formation
- Analyzer blockage
- Invalid results 1
Ice Packing Requirement
Rationale for Ice Transport
- Significant changes occur in ABG samples over time, especially at room temperature:
- pH decreases significantly after 30 minutes at room temperature vs. 45 minutes on ice
- PaO₂ decreases and PaCO₂ increases significantly from 15 minutes onward in both storage conditions
- These changes occur more rapidly and with greater magnitude at room temperature 3
Metabolic Activity in the Sample
- Cellular metabolism continues after collection:
- Ongoing oxygen consumption by blood cells reduces PaO₂
- Continued CO₂ production increases PaCO₂ and decreases pH
- Ice slows these metabolic processes, preserving sample integrity
Time Constraints
- Critical timing guideline: ABG samples should be analyzed within 15 minutes of collection regardless of storage method 3
- If analysis will be delayed beyond 15 minutes, ice packing becomes essential to minimize metabolic changes
Practical Implementation
Proper Collection Technique
For arteriovenous fistula/graft:
- Obtain blood from arterial needle before connecting blood tubing
- Ensure no saline or heparin is in the needle/tubing prior to drawing 4
For venous catheter:
- Withdraw heparin/saline from arterial port following protocol
- For adults: withdraw 10 mL of blood (3-5 mL for pediatric patients)
- Use a new syringe for the actual ABG sample 4
Common Pitfalls to Avoid
- Dilution with saline or excessive heparin (falsely lowers values)
- Inadequate anticoagulation (causes clotting)
- Delayed analysis without proper cooling
- Air bubbles in the sample (alter gas measurements)
- Drawing samples after dialysis has begun (for predialysis measurements)
By following these guidelines for heparin use and ice packing, healthcare providers can ensure accurate ABG results that truly reflect the patient's acid-base status and oxygenation, leading to appropriate clinical decisions and improved patient outcomes.