What is the difference between PTT (Partial Thromboplastin Time) and PT (Prothrombin Time) / INR (International Normalized Ratio)?

PTT vs PT/INR: Distinct Tests for Different Coagulation Pathways

PTT (Partial Thromboplastin Time) and PT (Prothrombin Time)/INR are fundamentally different coagulation tests that evaluate separate pathways: PTT monitors the intrinsic and common pathways (primarily for heparin therapy), while PT/INR monitors the extrinsic and common pathways (primarily for warfarin therapy). 1

What Each Test Measures

PT (Prothrombin Time) and INR

• PT measures the extrinsic coagulation pathway, specifically evaluating factors II, V, VII, X, and fibrinogen 2
• PT is reported in seconds but varies significantly between laboratories due to different thromboplastin reagent sensitivities 1
• INR standardizes PT results using the formula: INR = (PT patient/PT normal)^ISI, where ISI is the International Sensitivity Index of the thromboplastin reagent 1
• The INR was developed specifically because PT ratios varied markedly between laboratories, leading to excessive and erratic anticoagulation in North America where less responsive thromboplastins were commonly used 1

PTT (Partial Thromboplastin Time)

• PTT evaluates the intrinsic coagulation pathway, assessing factors VIII, IX, XI, XII, and the common pathway factors 2
• PTT is reported only as clotting time in seconds or as a ratio (patient-to-normal clotting time) 3
• Unlike PT, PTT has no standardized reporting system equivalent to INR 3

Primary Clinical Applications

When to Use PT/INR

• Monitoring warfarin (vitamin K antagonist) therapy - this is the primary and validated indication for INR reporting 1
• The INR therapeutic range of 2.0-3.0 (or 2.5-3.5 for mechanical valves) applies universally because of standardization 1
• Important caveat: INR is formally invalid outside of warfarin monitoring, though it is frequently misused as a universal harmonization system 3

When to Use PTT

• Monitoring unfractionated heparin therapy - PTT measures the therapeutic effect of IV heparin 4, 5
• The therapeutic goal is typically 1.5-2.5 times the baseline PTT value 5
• Normal PTT in a patient on unfractionated heparin suggests subtherapeutic anticoagulation 2

Critical Distinctions in Practice

Tests Should NOT Be Ordered Together Routinely

• Ordering both PT and PTT simultaneously is inappropriate in most clinical scenarios 4
• A study found that PT and PTT were invariably requested together despite lack of indication, with 232 unnecessary tests in just 50 patients (4.6 unneeded tests per patient) 4
• Specific indications:
  • Warfarin only: Order PT/INR alone 4
  • Unfractionated heparin only: Order PTT alone 4, 5
  • Low molecular weight heparin: Neither test is indicated for routine monitoring 4

Heparin Effects on Each Test

• Heparin primarily prolongs PTT, not PT - this is the basis for using PTT to monitor heparin therapy 5
• In patients achieving therapeutic aPTT ranges (approximately 75% by end of therapy), PT changes were not statistically significant 5
• During overlap periods when transitioning from heparin to warfarin, PT/INR guides warfarin dosing while PTT becomes less relevant 5

Common Pitfalls to Avoid

• Do not use INR for non-warfarin patients: The INR calibration model was developed using plasma from patients on stable warfarin for 6 weeks, making it less reliable in other contexts 1
• Do not use INR early in warfarin therapy: INR is less reliable during initiation, though still more reliable than unconverted PT ratios 1
• Do not monitor LMWH with either test: Low molecular weight heparin does not require routine PT or PTT monitoring 4
• Recognize lupus anticoagulant interference: Baseline PT in patients with lupus anticoagulant is rarely prolonged (unlike APTT) because PT reagents contain much higher phospholipid concentrations that quench LA activity 1