Do Not Collect VBG Samples in Blue-Top (Citrate) Tubes
Collecting a venous blood gas (VBG) sample in a blue-top citrate tube will significantly compromise the accuracy of your results and should be avoided—always use a blood gas syringe for VBG collection.
Why Citrate Tubes Are Inappropriate for Blood Gas Analysis
Blue-top tubes contain sodium citrate anticoagulant, which fundamentally alters blood gas parameters through two mechanisms:
Direct Effects of Citrate on Blood Gas Values
When blood is collected in evacuated citrate tubes rather than blood gas syringes, you will encounter:
- pH elevation: Mean positive bias of 0.03 units 1
- pCO₂ reduction: Mean negative bias of 5.0 mmHg 1
- HCO₃⁻ reduction: Mean negative bias of 1.2 mmol/L 1
- pO₂ elevation: Mean positive bias of 7.5 mmHg 1
These biases exceeded total allowable error thresholds in 16% of samples for pH, 40% for pCO₂, and 21% for HCO₃⁻ 1. This makes the results clinically unreliable for assessing ventilation and acid-base status.
Mechanism of Error
The inaccuracies stem from:
- Air contamination during transfer from the evacuated tube to analysis syringe
- Vacuum effects within the collection tube itself 1
- Dilution effects from the liquid citrate anticoagulant, which alters measured pCO₂ and base excess values 2, 3
A 2023 study confirmed that collecting peripheral venous blood in blood collection tubes (BCT) is unsuitable for pO₂, sO₂, fractional oxyhemoglobin, fractional deoxyhemoglobin, and oxygen content determinations 4.
Correct Collection Technique
Use dedicated blood gas syringes with dry heparin for all VBG samples 3. The key principles:
- Collect blood directly into the blood gas syringe by venipuncture
- Avoid any transfer steps that expose blood to air
- Use 19-21 gauge needles (no difference in values) 5
- Apply tourniquet for ≤60 seconds only 5
- Minimize dead space filling with any diluent 3
Storage Considerations
If immediate analysis is not possible:
- Room temperature: Up to 30 minutes for pH/pCO₂/HCO₃⁻; only 12 minutes for pO₂ 2
- Ice-water bath: Up to 3.5 hours for pH; up to 6 hours for pCO₂ and pO₂ 2
Common Pitfall
The most critical error is assuming that any anticoagulated tube is acceptable for blood gas analysis. Citrate tubes are designed for coagulation studies, not blood gas analysis 5, 6. Even minimal dilution from liquid anticoagulants produces unacceptable results 3.