Test Overview
Test Methodology

Photo-optical clot detection using recombinant thromboplastin.

Test Usage

The PT is sensitive to a factor VII deficiency and it is moderately sensitive to deficiencies of factors II, V, X and fibrinogen. The PT assay can also be used to determine, Warfarin (Coumadin) therapy, vitamin K deficiencies, liver disease, and intravascular coagulation syndrome (DIC).

Reference Range *

9.4 - 12.2 seconds. Critical value: INR >5.0. Recommended therapeutic ranges for oral anticoagulant therapy: INR 1.2 - 1.5 Prevention of catheter-related venous thrombosis; INR 2.0 - 3.0 Treatment of venous or pulmonary embolism, prevention of systemic embolism, tissue heart valves, atrial fibrillation, valvular heart disease; INR 2.5 - 3.5 Mechanical prosthetic valves; INR >2.0 - 3.0 Certain patients with thrombosis and the antiphospholipid syndrome.

* Reference ranges may change over time. Please refer to the original patient report when evaluating results.

Test Limitations

A large number of drugs can interfere with the action of warfarin in vivo, either potentiating or inhibiting its effect on the coagulation factors II, VII, IX, and X. The Prothrombin Time is insensitive to unfractionated heparin up to approximately 2.0 units per mL. Inhibitors such as the lupus anticoagulant may interfere with Prothrombin Times. Direct thrombin inhibitors (Argatroban, Bivalarudin, etc) in therapeutic doses will result in prolonged Prothrombin Times.Patients with abnormally elevated hematocrits may show falsely prolonged PT.

Test Details
Days Set Up
Daily, 24 hours
Analytic Time

STAT 1 hour, Routine 4 hours

Soft Order Code
MiChart Code
  • INR
  • International Normalized Ratio (INR)
  • Pro Time
  • Prothrombin Time
  • Protime-INR
  • PT and INR
STAT Availability

STAT requests for this test will be performed on a STAT basis (supervisory staff approval is not required).

Specimen Requirements
Collection Instructions

Collect specimen in a blue top (citrate 3.2%) tube. Mix by inversion. Specimen should arrive at lab within 23 hours of collection; transport at room temperature. Alternatively, centrifuge, aliquot plasma into a plastic tube, and freeze the specimen within 4 hours of collection. Transport frozen specimen on dry ice. Collection of the blood through lines that have been previously flushed with heparin should be avoided. If the blood must be drawn through a VAD (vascular access device), the line should be flushed with 5 mL of saline and the first 5 mL of blood or six dead space volumes of the VAD discarded.

Normal Volume
Full 2.7mL tube or 1mL plasma aliquot
Minimum Volume
Full 1.8mL tube or 1mL plasma aliquot
Storage Temperature
Room temperature for whole blood or frozen aliquot of citrated plasma
Rejection Criteria
Specimens will be rejected if not properly filled, clotted, grossly hemolyzed,contaminated with heparin,or refrigerated.
Additional Information

The Prothrombin time in seconds is reported along with the corresponding International Normalized Ratio (INR).

CPT Code
Fee Code