6.2 Prothrombin Time and International Normalized Ratio

• What does prothrombin time indicate?

  Concentration of prothrombin in blood
• How is prothrombin time determined?

  By measuring coagulation time after adding calcium and tissue factor
• What is the normal prothrombin time?

  About 12 seconds
• Why is prothrombin time variable in individuals?

  Due to differences in tissue factor activity
• What is the purpose of the international normalized ratio (INR)?

  To standardize prothrombin time measurements
• What does the international sensitivity index (ISI) indicate?

  Activity of the tissue factor with a standard sample
• How is the INR calculated?

  INR = $\left(\frac{PT_{test}}{PT_{normal}}\right)^{ISI}$
• What is the normal range for INR in a healthy person?

  0.9 to 1.3
• What does a high INR level indicate?

  High risk of bleeding
• What does a low INR level suggest?

  Chance of having a clot
• What INR range do patients undergoing warfarin therapy usually have?

  2.0 to 3.0
• What is the process for determining other blood clotting factors similar to prothrombin time?

  • Add excess calcium ions and other factors to oxalated blood
  • Measure coagulation time
  • Prolonged time indicates deficiency of the tested factor
• How does the presence of excess calcium ions affect the prothrombin time test?

  It nullifies the effect of oxalate
• What role does tissue factor play in the prothrombin time test?

  It activates the prothrombin-to-thrombin reaction
• From where is tissue factor isolated?

  Human tissues, such as placental tissue
• Why might different batches of tissue factor have varying activity?

  Due to differences in isolation methods
• What can prolonged coagulation time indicate in the context of blood clotting factor tests?

  Deficiency of the tested clotting factor