What is the proper technique for obtaining a potassium blood sample from a line that has been flushed with heparin?

Proper Technique for Drawing Potassium from a Heparin-Flushed Line

When drawing a potassium sample from a line flushed with heparin, you must discard at least 3 times the dead space volume (ideally 5 times) before collecting your sample to prevent contamination that could falsely alter results. 1

Why This Matters

Heparinized saline contamination creates artefactually increased phosphate concentrations and can affect other electrolyte measurements 1. While the guidelines primarily discuss glucose contamination (which is catastrophic), the same principles of dead space removal apply to all flush solutions including heparinized saline to prevent sample contamination 1.

The Technique: Open vs. Closed Systems

Open System (Single Three-Way Tap)

• The traditional recommendation is to discard 3 times the dead space volume 1
• However, bench-top experiments demonstrate that even 5 times dead space removal may not completely eliminate contamination with certain flush solutions 1
• The dead space is the volume between your sampling point and the bloodstream
• Critical pitfall: Using the same port for both discarding and sampling increases contamination risk

Closed System (Preferred)

• Has a separate port for dead space removal beyond the sampling point 1
• Effectively eliminates significant contamination risk when used correctly 1
• Reduces bacteremia risk and minimizes blood wastage since withdrawn blood can be returned 1

Specific Recommendations for Potassium Sampling

For peripheral venous catheters with heparin flush, research shows you should discard a volume equal to 2 times the dead space of the catheter and extension set for accurate coagulation studies 2. However, given that potassium measurements from peripheral catheters show wider variation than venipuncture (±0.37 mmol/L vs ±0.21 mmol/L), and this exceeds the acceptable interval of ±0.3 mmol/L 3, obtaining potassium from peripheral catheters is not recommended as an acceptable alternative to venipuncture 3.

Key Caveats

• Heparin concentration matters: Increasing heparin concentrations and volumes can actually decrease measured potassium values 4
• Watch for erratic results: Highly varying sequential test results should immediately raise suspicion of sample contamination 1
• Gauge size affects success: 18-gauge catheters have 78% success rate vs 42% for 20-gauge when drawing samples 3
• Never draw from lines carrying other solutions: Blood sampling from a cannula lumen that carries solutions other than sodium chloride 0.9% (with or without heparin) is not recommended 1

The Bottom Line Algorithm

1. First choice: Use a closed system with separate discard port - discard 3-5× dead space
2. Second choice: Use open system - discard minimum 5× dead space volume
3. Best practice: Avoid drawing potassium from peripheral catheters entirely; use venipuncture instead 3
4. If results seem off: Repeat with venipuncture to confirm