Can prothrombin time (PT) and partial thromboplastin time (PTT) be drawn from a central line?

Drawing PT/PTT from Central Lines: Technical Feasibility with Critical Caveats

PT and PTT can technically be drawn from central lines, but the accuracy depends critically on the type of central line, proper technique with adequate discard volumes, and whether the line is being used for heparin infusion.

Key Recommendations by Line Type

PICCs (Peripherally Inserted Central Catheters)

• PICCs are generally acceptable for PTT measurement when proper technique is used 1
• A 2007 study demonstrated no clinically significant differences between aPTT values drawn through PICCs versus peripheral venipuncture 1
• Adequate discard volume is essential (see below)

Implantable Ports

• Avoid drawing PTT from implantable ports - these consistently yield inaccurate results 2
• Root cause analysis at a major cancer center revealed multiple critically elevated PTT levels from ports were false results 2
• Use peripheral venipuncture instead for coagulation studies when ports are the only central access 2

Arterial Lines and Other Heparinized Catheters

• Can be used but require strict adherence to discard volumes 3, 4
• Minimum 5.3 mL discard volume required for accurate PT/PTT from arterial lines 3
• With only 3 mL discard, significant differences occur in both PT and aPTT values 3
• The heparin concentration in flush solution matters: 2 units/mL requires larger discard than 1 unit/mL 4

Critical Contraindication

Never draw coagulation studies from a line actively infusing heparin, even with discard volumes 5:

• A 2000 study comparing 10 mL vs 20 mL discard volumes from heparin-infusing catheters found statistically significant differences in both groups 5
• Even 20 mL discard showed persistent contamination 5
• Always use direct venipuncture or a peripheral catheter in the opposite arm when monitoring heparin therapy 5

Proper Technique When Drawing from Central Lines

Essential steps to minimize contamination:

1. Stop any infusions through the line temporarily
2. Discard at least 5-7 mL of blood (beyond the dead space volume)
3. Draw the coagulation sample
4. For arterial lines with 1 unit/mL heparin flush: minimum 5 mL discard above dead space 4
5. For arterial lines with 2 units/mL heparin flush: even larger discard volumes needed 4

Common Pitfalls to Avoid

• Insufficient discard volume is the most common error leading to falsely elevated results
• Drawing from the wrong type of line (especially implantable ports for PTT)
• Attempting to draw from heparin infusion lines regardless of discard volume
• Not accounting for catheter dead space in discard calculations
• Assuming all central lines are equivalent (they are not)

Clinical Context from Guidelines

While the 2025 Association of Anaesthetists guidelines 6 and 2015 ASA guidelines 7 provide PT/PTT thresholds for managing central line insertion and removal, they do not specifically address the technical question of drawing these labs from central lines—this is a separate procedural consideration that requires the evidence-based approach outlined above.