Prothrombin Time (PT) is a critical coagulation test used to assess the extrinsic pathway of blood clotting. However, labs report PT in different ways—seconds, percentage (PT%), or INR (International Normalized Ratio)—leading to confusion. This guide explains:

• How PT is measured and reported

• Why different methods exist

• How to compare results between labs

• Which method is most reliable

• What it is: The actual time (in seconds) it takes for blood to clot after adding thromboplastin.

• Normal range: 11–14 seconds (varies by lab and reagent).

• Pros:

  • Direct measurement, no calculation needed.

  • Useful for general clotting assessment.

• Cons:

  • Varies between labs due to different reagents/analyzers.

• What it is: An old method expressing clotting activity as a percentage of normal plasma.

• Normal range: ~70–130% (depends on calibration curve).

• Pros:

  • Historically used before INR standardization.

• Cons:

  • Highly variable (depends on lab-specific curves).

  • No longer reliable—replaced by INR.

• What it is: A standardized PT value adjusted for reagent differences.

• Formula:

  $$INR={\left(\genfrac{}{}{0ex}{}{\text{Patient PT}}{\text{Mean Normal PT (MNPT)}}\right)}^{\text{ISI}}$$

  • ISI (International Sensitivity Index): Reagent-specific factor (lower = more sensitive).

• Normal range: 0.9–1.1

• Therapeutic range (warfarin): 2.0–3.0 (or higher for certain conditions).

• Pros:

  • Consistent across labs (best for monitoring warfarin).

• Cons:

  • Only reliable for vitamin K antagonists (e.g., warfarin).

• PT% is obsolete and should not be used for clinical decisions.

• INR is the only reliable way to compare PT across different labs.

• Lab A: Reports PT = 7.7%, INR = 0.64

• Lab B: Reports PT = 13.6 sec

1. Check if INR is available (most reliable).

   • If Lab A’s INR = 0.64, it suggests faster-than-normal clotting.

   • If Lab B reports PT = 13.6 sec, calculate its expected INR (if MNPT = 12 sec, ISI = 1.0):

     $$INR={\left(\genfrac{}{}{0ex}{}{13.6}{12}\right)}^1\approx 1.13(\text{normal})$$

   • Conflict: Lab A’s INR (0.64) ≠ Lab B’s expected INR (~1.1).

2. If only PT% is given (unreliable):

   • PT% 7.7% suggests severely prolonged PT, but this contradicts Lab A’s low INR.

   • Conclusion: PT% is misleading—ignore it.

3. Final Verdict:

   • Trust INR > PT (seconds) > PT%

   • If INR is missing, ask the lab to recalculate it.

✔ INR is the only standardized method – use it for warfarin and cross-lab comparisons.
✔ PT (seconds) is useful but lab-dependent – always check the reference range.
❌ PT% is outdated – do not use it for clinical decisions.

1. Check if INR is reported (if not, request it).

2. Verify the lab’s reference ranges for PT (seconds).

3. Repeat the test if INR/PT values seem contradictory.

4. Consult a hematologist if discrepancies persist.

• For warfarin patients: INR is mandatory.

• For general clotting tests: PT (seconds) is acceptable, but INR is better for consistency.

• Never rely on PT% – it’s obsolete and unreliable.

By understanding these differences, you can avoid misinterpretation and ensure accurate coagulation monitoring.

Need further clarification? Ask in the comments! 🚀