Normalized Ratio (INR)
PROTHROMBIN TIME (089-0106)
Synonyms: PT, Pro-Time
CPT 4 Code: 85610
Test Order Mnemonic: PT
Applies To: Useful in monitoring
Warfarin/Coumadin therapy and detecting abnormalities of the extrinsic clotting
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: Viable for 24 hours at room
temperature. 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
Causes for Rejection: QNS, clotted, severely hemolyzed
specimen, specimen greater than 24 hours old, identification error, sample
processed 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
Special Instructions: Deliver sample to lab within ½
hour for best results. Accepted up to 24 hours.
Volume: 4.5 ml
Min Volume: Call Lab
Container: 3.2% Sodium citrate tube (light Blue top
Reference Range: 11.7 - 13.5 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 permited development of effective recommendations for use of oral
anticoagulants in a variety of clinical setings. 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
(and/or contact laboratory for further
Critical Value: PT > 24 seconds; INR >
Test Method: Photo-Optical Detection
Interfering Substances: Anticoagulant
therapy, barbiturates, oral contraceptives, anabolic steroids.
LABORATORY MONITORING OF ORAL ANTICOAGULANT THERAPY
RECOMMENDED LABORATORY TEST
The Prothrombin Time (PT) is
the laboratory test of choice for monitoring the anticoagulation status of
patients treated with oral anticoagulants (Coumarin, Warfarin, Coumadin).
THERAPEUTIC RANGE USING THE INR
The activity of the thromboplastin reagent used in the
Prothrombin Time (PT) test varies from lot to lot. 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. 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.
FREQUENCY OF MONITORING
INITIATION PHASE: During the initiation phase of oral
anticoagulant therapy, the patient's status should be monitored 4-5 times per
week until some degree of consistency in the INR response is noted.
STABLE PHASE: Once the anticoagulant dose and INR response
stabilizes the INR should be determined on an individual patient basis. But, in
general, the INR should be monitored at least once each 4 weeks.
TRANSITION PHASE: A transition phase occurs whenever there
are changes to medication or medical condition. A variety of drugs as well as
significant dietary changes can significantly alter the response to oral
anticoagulant therapy. Therefore, it is very important to more closely monitor
the INR when patients in the stable phase experience changes to medication or
CONCURRENT HEPARIN THERAPY: During periods in which
patients are treated with both heparin and an oral anticoagulant, the effect of
heparin upon the Prothrombin Time (and INR) can lead to over-estimation of the
therapeutic level of oral anticoagulation. During this time, it is important to
closely monitor the INR during the transition to oral anticoagulation especially
if this time occurs during the early outpatient period.
- The presence of anti-phospholipid antibodies (lupus anticoagulants) can
alter the PT and give rise to INR values that can result in incorrect dosing.
Contact the laboratory for recommendations if a lupus anticoagulant is suspected.
- The INR is specifically intended to improve the management of orally
anticoagulated patients. Since it is not known how well the INR correlates with
diagnosis or outcome in other clinical settings its use in these situations
should be restricted. In most other clinical settings in which identification of
defects in the extrinsic clotting cascade is important, the Prothrombin Time
(PT) is likely to be the more appropriate test.
Recommended Therapeutic Range for Warfarin/Coumadin
INTERNATIONAL NORMALIZED RATIO
|Treatment of venous
2.0 - 3.0
|Treatment of pulmonary
|Prevention of systemic
2.5 - 3.5
thrombolitic disorders i.e.. Antiphospholipid
plus Prothrombin 20210A mutation and history of
the case hypothesised an INR of 3.75 would prove beneficial)
*Hirsh J, Dalen JE, Deykin D, Poller L, Bussey H. Oral anticoagulants:
mechanism of action, clinical effectiveness, and optimal therapeutic range.
Fourth ACCP Consensus Conference on Antithrombotic Therapy. Chest. 1995;
+Many investigators believe that antiphospholipid antibodies are an
indication for high-intensity warfarin therapy.
Added by E. McLucas MT(ASCP), 05/31/2001