Return to Test Directory
Prothrombin Time (089-0106)
Test Mnemonic:


Specimen Requirements:

Blue-topped (3.2% sodium-citrate) tube. Routine venipuncture; discard 1st mL of blood by collecting a discard tube prior to collecting the blue-topped (3.2% sodium-citrate) tube.  For collections with butterfly blood collection sets, a discard tube should also be collected prior to collection of the blue top to ensure sufficient sample volume. Drawing a discard tube will displace the air from the blood collection set tubing to ensure proper blood draw volume.


Blue-topped (3.2% sodium-citrate) tube


If time from draw to receipt in the laboratory is to be greater than 2 hours, centrifuge the specimen for 15 minutes at 2000-2500g.  Remove the plasma from red cells.


Specimen Preparation:

Samples drawn for Prothrombin Time testing should be stored and transported at ROOM TEMPERATURE to avoid activation of Factor VII.


Unrefrigerated whole blood samples are viable for 2 hours.

Plasma may be stored at 20+/- 5°C for 8 hours.

Plasma may be double spun and frozen at -20 °C for up to 2 weeks. Please click here for instructions on how to perform the double spin technique.  

Causes for Rejection:

QNS, clotted, severely hemolyzed specimen, specimen greater than 24 hours old, wrong tube (3.8% sodium citrate), high hematocrit (> 55%), sample identification error, sample processed/transported, and/or stored improperly.

Reference Range:

12.0 – 14.7 seconds


Galveston & Clear Lake Campus reference range: 10.1 - 12.6 secs

Normal INR <1.1; warfarin Therapeutic range: 2.0 to 3.0 or 2.5 to 3.5, depending upon the indications

Turnaround Time:

STAT: 1 hour; ROUTINE: 4 hours


Electromagnetic Viscosity Detection


 Samples are accepted 24 hours per day at Sample Management, 7.412 CSW Bldg.




Clinical Indication:

Useful in monitoring Warfarin/Coumadin anticoagulation therapy and detecting abnormalities of the extrinsic clotting pathway.

CPT 4 Code:



Therapeutic Range:  Using the INR: Standardization of the Prothrombin Time (PT) with the International Normalized Ratio (INR) allows for uniform measurement of the anticoagulation status of patients on oral anticoagulants (Warfarin/Coumadin/Coumarin).  The use of the INR has permitted development of effective recommendations for use of oral anticoagulants in a variety of clinical settings.  Most published studies indicate that, in most cases, an INR of at least 2.0 is required for effective anticoagulation.  The risk of bleeding increases with an increasing INR, and may increase dramatically above an INR of 4.5 - 5.0.   Refer to Monitoring Oral Anticoagulant Therapy or contact laboratory [409-772-3314] for further recommendations.

Critical Values:  INR > 4.5

Interfering Substances: Heparin above therapeutic ranges, afibrinogenemia or hypofibrinogenemia

When ordering tests for which Medicare or Medicaid reimbursement will be sought, physicians should only order tests that are medically necessary for the diagnosis or treatment of the patient. Components of the organ or disease panels may be ordered individually. The diagnostic information must substantiate all tests ordered and must be in the form of an ICD-10 code or its verbal equivalent.
Return to Test Directory