How do I order a Prothrombin Time (PT)/International Normalized Ratio (INR) test?

How to Order PT/INR Testing

To order a PT/INR test, submit a standard laboratory requisition form specifying "PT/INR" and collect a blood sample in a light blue-top tube containing sodium citrate, ensuring proper fill volume to maintain the correct blood-to-anticoagulant ratio. 1, 2

Sample Collection Requirements

• Tube type: Light blue-top tube containing 3.2% sodium citrate
• Fill volume: Must be filled to the indicated line (maintaining 9:1 blood-to-anticoagulant ratio)
• Timing: Can be collected at any time of day without significant variation in results 3
• Processing: Sample should be processed within 4 hours of collection
• Special considerations:
  • Avoid drawing from an arm with an IV line
  • If collecting multiple tubes, collect the blue-top tube after any non-additive tubes but before EDTA tubes

Clinical Indications for PT/INR Testing

1. Monitoring vitamin K antagonist therapy (e.g., warfarin) 2

   • Standard therapeutic range: INR 2.0-3.0 for most indications
   • Testing frequency: Daily after initial dose until stabilized, then every 1-4 weeks

2. Pre-procedural coagulation assessment

   • Recommended just prior to dental or surgical procedures in patients on anticoagulants 2

3. Evaluation of liver function

   • Note: Standard INR has limitations in liver disease; consider INR-liver (modified for cirrhosis) 4

Understanding PT/INR Results

• Normal range: INR 0.8-1.2 (may vary slightly by laboratory)
• Therapeutic range for warfarin: INR 2.0-3.0 for most indications 2
• INR >4.0: Associated with higher bleeding risk without additional therapeutic benefit 2

Important Considerations and Pitfalls

• The INR scale is only validated for patients on vitamin K antagonists with no other superimposed coagulation defect 4

• PT/INR has limitations for monitoring coagulation in:

  • Patients with liver disease (consider INR-liver) 4, 1
  • Patients with lupus anticoagulant (may falsely elevate INR) 1
  • Patients on direct oral anticoagulants (DOACs) 1

• Avoid ordering plasma transfusion based solely on mildly elevated INR (1.0-2.0) in non-bleeding patients not on anticoagulants, as there is no evidence of clinical benefit 4

Laboratory Standardization

The INR is calculated using the formula: INR = (patient PT/normal PT)^ISI, where ISI is the International Sensitivity Index specific to the laboratory's thromboplastin reagent and instrument 4, 5. This standardization allows for consistent monitoring across different laboratories.

By following these guidelines for ordering PT/INR testing, you can ensure accurate results for appropriate clinical decision-making while avoiding unnecessary testing or interventions.