PROTHROMBIN TIME (089-0106)

Synonyms: PT, Pro-Time

CPT Code(s):  85610  Limited Coverage Test (NCD)

Test Order Mnemonic:  PT

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

Test Includes:  Patient results, INR (International Normalized Ratio)

Request Form:  Hematology A

Collection:  Routine venipuncture; discard 1st mL of blood or collect other tubes (EDTA, red-topped) prior to collecting sample in blue-topped (citrate) tube.

Storage Instructions:  Samples drawn for Prothrombin Time testing should be stored and transported at ROOM TEMPERATURE to avoid activation of Factor VII.  Unrefrigerated samples are viable for 24 hours.  

NOTE:  If time from draw to receipt in the laboratory is to be greater than 24 hours, centrifuge the specimen; remove the plasma from red cells and snap freeze (-70°C) plasma. Snap frozen plasma is viable for 6 months.

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

Availability: Samples are accepted 24 hours per day at Sample Management, 5.136 McCullough Bldg.

Turnaround Time:   STAT: 1 hour; ROUTINE: 4 hours

Special Instructions: Store and transport specimen at ROOM TEMPERATURE.  Deliver sample to lab within ½ hour for best results. Accepted up to 24 hours.

Specimen:  Blood

Volume:  4.5 ml

Minimum Volume:  Call Lab

Container:  3.2% Sodium citrate tube (Light Blue top tube)

Reference Range:  12.0 – 14.7 seconds

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 [http://www.utmb.edu/lsg/LabSurvivalGuide/hem/ORAL_ANTICOAG_THERAPY.htm] or contact laboratory [409-772-3314] for further recommendations.

Critical Values:   PT > 42.1 seconds (Changed 12/10/07)

Test Methodology:  Electromagnetic Viscosity Detection

Interfering Substances:  Heparin above therapeutic ranges, afibrinogenemia or hypofibrinogenemia