How to order Prothrombin time (PT/INR)?

How to Order Prothrombin Time (PT/INR)

To order Prothrombin Time (PT/INR), request a "PT/INR" test using a blue-top (sodium citrate) tube, collected with proper technique to avoid pre-analytical errors. 1

Sample Collection Requirements

• Use a blue-top tube containing 3.2% sodium citrate
• Fill tube completely to ensure proper blood-to-anticoagulant ratio (underfilling causes falsely prolonged results)
• If drawing multiple tubes, collect blue-top tube after plain tubes but before EDTA tubes
• Avoid hemolysis, which can affect results
• Process sample within 4 hours of collection

Clinical Applications

PT/INR testing is primarily used for:

1. Monitoring vitamin K antagonist (VKA) therapy such as warfarin

   • Target INR ranges typically 2.0-3.0 or 2.5-3.5 depending on indication 1
   • For patients on warfarin, timing of blood draws should be consistent relative to dosing

2. Evaluating liver function

3. Assessing bleeding disorders

4. Screening for factor deficiencies (II, V, VII, X)

Interpretation Considerations

• Normal PT values typically range from 10-13 seconds

• INR is calculated using the formula: INR = (PT/MNPT)^ISI 1

  • MNPT = Mean Normal Prothrombin Time
  • ISI = International Sensitivity Index

• Important caveat: INR was specifically designed and validated only for monitoring VKA therapy, not for general coagulation assessment 1

Monitoring Frequency

For patients on VKA therapy:

• Initially: Daily testing until stabilization in therapeutic range
• Once stable: Testing intervals typically range from one to four weeks
• Additional testing when changing between warfarin products or starting/stopping other medications 2

Common Pitfalls to Avoid

1. Pre-analytical errors:

   • Underfilled collection tubes
   • Contamination with tissue factor (traumatic venipuncture)
   • Delayed sample processing

2. Misapplication of INR for purposes other than VKA monitoring

   • Different thromboplastins have varying sensitivities to coagulation factor deficiencies 1

3. Failure to consider the time between DOAC administration and blood sampling when interpreting results 1

4. Using large loading doses of warfarin, which may increase bleeding risk without offering more rapid protection 2

By following these guidelines for ordering and interpreting PT/INR tests, clinicians can ensure accurate results that properly inform clinical decision-making and optimize patient outcomes.