What is the difference between PT, INR and PTT?

PT (Prothrombin Time), INR (International Normalized Ratio), and PTT (Partial Thromboplastin Time) are all laboratory tests used to evaluate different aspects of blood coagulation. Here is a breakdown of each:

PT (Prothrombin Time)
Purpose: Measures the time it takes for blood to clot.
Pathway: Evaluates the extrinsic and common pathways of the coagulation cascade.
Factors Assessed: Primarily factors II (prothrombin), V, VII, and X.
Common Uses: Monitoring warfarin (Coumadin) therapy, diagnosing liver disease, and investigating bleeding disorders.
INR (International Normalized Ratio)
Purpose: Standardizes PT results regardless of testing methods or reagents used across different laboratories.
Calculation: INR = (Patient PT / Control PT)^ISI, where ISI is the International Sensitivity Index assigned to the reagents used.
Common Uses: Primarily used to monitor patients on warfarin therapy to ensure consistent and safe anticoagulation levels.
PTT (Partial Thromboplastin Time)
Purpose: Measures the time it takes for blood to clot.
Pathway: Evaluates the intrinsic and common pathways of the coagulation cascade.
Factors Assessed: Factors I (fibrinogen), II (prothrombin), V, VIII, IX, X, XI, and XII.
Common Uses: Diagnosing hemophilia, monitoring heparin therapy, and investigating unexplained bleeding or clotting disorders.
Summary of Differences
Pathways:
PT/INR: Extrinsic and common pathways.
PTT: Intrinsic and common pathways.
Use in Medication Monitoring:
PT/INR: Warfarin.
PTT: Heparin.
Standardization:
PT: Can vary by lab and reagent.
INR: Standardized version of PT.
PTT: Not standardized like INR, but used consistently for intrinsic pathway evaluation.
Each test has a specific role and is chosen based on the clinical context, such as monitoring anticoagulant therapy or diagnosing bleeding disorders.