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Test Code Prothrombin time Prothrombin time

Important Note

This is recommended for vitamin K antagonists (eg, warfarin) monitoring.

Direct oral anticoagulants (non-vitamin K) should not be monitored with this assay.

Tube must be properly filled.

Additional Codes

Cerner

NextGen

Protime

Protime/ INR

PT

 

Alternate Name(s)

PT

INR

Useful For

The Prothrombin time (PT) test is a basic coagulation screening test that measures, as a whole, the activity of Factors I (fibrinogen), II (prothrombin), V (proaccelerin), VII (proconvertin), and X (Stuart factor). Prolonged PT has been observed in the following conditions: Congenital or acquired deficiencies of Factors I, II, V, VII, X; liver failure associated with cirrhosis and hepatitis; treatments with Vitamin K antagonists (e.g., Warfarin); hypovitaminosis K due to nutritional deficiency or disorders in absorption or metabolism of vitamin K (caused by hemorrhagic disease of the newborn, cholestasis, antibiotic treatment); fibrinolysis; DIC. The PT test is also used to monitor Vitamin K antagonist therapy because of its sensitivity to variations in the concentration of the Vitamin-K dependent factors II, VII and X.

Methodology

The STA Compact Max® is a fully automated coagulation instrument which uses an electromagnetic mechanical clot detection system.  A mixture of thromboplastin is added to citrated plasma and the time for clot formation is determined, which is compared to that of a known standard. The oscillation of a steel ball within the cuvette containing the thromboplastin and plasma is monitored by the STA Compact Max®.  When the oscillation of the steel ball is slowed by clot formation, reaching an endpoint, the sensor determines the time in seconds.

Patient Preparation

None

Collection Instructions

  • Sample collections must comply with the recommendations for hemostasis tests.
  • 3.2% sodium citrate tubes only.
  • Tube must be properly filled.
  • Do not refrigerate specimens.

Specimen Requirements

Preferred Volume

1 mL plasma

Minimum Volume

1 mL plasma

Container

3.2% sodium citrate tube (blue top), BD or Greiner Vacutainer tubes

Stability
  • Do not refrigerate specimens.
  • Centrifuged or uncentrifuged specimens: 24 hours at 18-24⁰ C.
  • Separated plasma:
    • 24 hours at 18-24⁰ C.
    • Platelet poor plasma is stable for 2 weeks at -20⁰ C. Specimen must be double spun and separated prior to freezing.

Rejection Criteria
  • Clotted
  • Hemolyzed
  • Expired collection tube
  • Under or Overfilled tube (blood/anticoagulant ratio must be 9:1; tube filled between 90-110 %)
  • Improperly mixed specimen    

Result Reporting and Reference Values

Reference Range

PT     11.9-14.3 seconds

INR    0.9 – 1.1

Therapeutic Range

INR    2.0 – 3.0 (standard dose)

INR    2.5 – 3.5 (high dose)

Critical Ranges

INR    > 5.0

Reflex Testing

None

Limitations

  • The slightest micro-clots in a specimen will induce considerable shortening of the times measured (autocatalytic activation of all the factors) whereas extensive coagulation will prolong the clotting times because of consumption of factors and fibrinogen.
  • Thrombin inhibitors (e.g. hirudin, argatroban, etc.) present in the sample to be tested may lead to a prolonged prothrombin time.