Spectralaboratories.com

Prothrombin Time
Introduction
The Prothrombin Time (PT) was first described by Quick et al. in 1935. Oral
anticoagulants were initial y introduced for clinical use in the 1940’s, and since then a variety of laboratory tests have been used to monitor therapy. Five decades later, PT testing is the most commonly used test to monitor oral anticoagulants. Prothrombin Time measures the effect of a reduction in the vitamin K-dependent coagulation Factors I , V, VI and X, and it measures the integrity of the extrinsic system.
Prothrombin Time is performed by adding a mixture of calcium and thromboplastin to the test plasma. The length of time, in seconds, it takes for the sample to clot is the Sample Collection
Buffered citrate is the standard anticoagulant for col ection of samples for coagulation studies. Spectra Laboratories provides the BD light blue hemogard top tube, .7 mL draw—a sterile tube with siliconized interior, with 0.109M buffered • International Normalized Ratio (INR) sodium citrate (equivalent to 3.% sodium citrate). The tube is labeled with distinct yellow The optimal volume ratio of anticoagulant to blood is 1:9. It is extremely important that this ratio is maintained when obtaining blood for PT or Activated Partial Thromboplastin Time (APTT). The amount of anticoagulant to blood increases if the tube is not filled to vacuum capacity, causing falsely prolonged PT and APTT. It is important that Oral Anticoagulant - Coumadin
the tube be mixed by gentle inversion soon after Coumadin and other coumarin anticoagulants act by blood collection. Inadequate mixing causes the inhibiting the synthesis of vitamin K-dependent clotting sample to clot, rendering it unsuitable for testing. factors which include Factors II, V, VII, X and the Do not shake; shaking can cause activation of some anticoagulant proteins C and S. Coumadin (sodium warfarin) is primarily used to treat many thromboembolic (clot forming) conditions. The treatment is monitored closely because the anticoagulant response to fixed dosages varies among individuals. The efficacy and safety of warfarin are highly dependent on maintaining the anticoagulant effect within a defined therapeutic range. Coumadin is considered a “narrow therapeutic index drug,” meaning that there is a very narrow window where coumadin is considered safe and effective. The most serious risk associated with anticoagulant therapy with sodium warfarin is hemorrhage in any tissue or organ. The risk of hemorrhage is related to the level of intensity and the duration of anticoagulant therapy.
Heparin Extracted Prothrombin Time
To better serve dialysis patients on oral anticoagulant
therapy, Spectra Laboratories has validated the use of heparin extraction reagent to neutralize heparin-contaminated samples for PT testing. An anion exchange resin is added to the plasma and binds the heparin. The heparin-cellulose complex is • Incorrect International Sensitivity Prothrombin Time results equal to or greater than 60 seconds on initial testing, will be retested utilizing the heparin extraction procedure. This procedure effectively absorbs and removes up to U of heparin per mL of plasma, producing accurate PT results International Normalized Ratio
Recognizing the inherent variables and limitations of the
Prothrombin Time Testing Interference
PT assay as an effective monitor of oral anticoagulant One of the perplexing problems in coagulation therapy, the World Health Organization (WHO), along testing is heparin contamination of blood samples. with the International Committee on Thrombosis Heparin is used prophylactical y during dialysis to and Haemostasis (ICTH) and the International maintain the patency of the extracorporeal circuit. Committee for Standardization in Haematology It is also instilled in the ports of central venous (ICSH), developed a protocol to standardize the PT. catheters intradialytically to prevent clotting The protocol is based upon the establishment by WHO of one of the ICTH thromboplastin preparations as the international reference against which all other It is recommended that at least 10 mL of blood
thromboplastins would be calibrated. The end result be removed from each port of the central venous
was the creation of the International Normalized Ratio catheter prior to the collection of blood for
(INR) calculation that effectively equates any source PT analysis, to avoid the pitfalls of heparin
of thromboplastin to the WHO “gold standard.” contamination.
Specimens contaminated with residual heparin produce PT results of questionable value. It also delays reporting of accurate results and additional costs are incurred in performing repeat testing.
The INR is derived in the following way: the patient PT, in seconds, is divided by the mean PT of the normal population. This ratio, referred to as the Prothrombin Time Ratio (PTR), is then raised to the power of a value known as the International the clinical situation of the patient.
Sensitivity Index (ISI). The result of the PTR taken to the power of the ISI value is called the International Normalized Ratio, or INR. Table 1
Recommended Therapeutic Ranges for
The ISI value represents the responsiveness of the PT to the reduction of vitamin K-dependent coagulation Factors II, V, VII and X, measured with a given thromboplastin reagent used in the assay. The WHO reference reagent was assigned an ISI of 1.0. Therefore, thromboplastins as sensitive as the WHO reference reagent will also have an ISI of 1.0. Relatively less sensitive thromboplastins wil have an ISI greater than 1.0. The higher the ISI, the less sensitive the thromboplastin. The less sensitive the thromboplastin, the shorter the PT result. The INR value is then used by the physician to manage the drug dosage, relative to the clinical situation of the patient.
INR Value
The standardized INR system is intended only for patients on stable oral anticoagulant therapy. The PT values from patients with other coagulation Hirsh J, Dalen JE, Deykin D, Pol er L. Oral anticoagulants: mechanism of defects often do not follow the relationship predicted action, clinical effectiveness, and optimal therapeutic range. Chest. 199; 10 (suppl): 31S-36S by the ISI and INR. The Sixth American College of Chest Physicians (ACCP) Consensus Conference on Antithrombotic Therapy (000) provided recommended therapeutic ranges for therapy with oral anticoagulants. (Table 1) Normal Population Mean
Normal population range (reference range) consists of thromboplastin with a low ISI value provide a wider data generated from a group of individuals judged to be range (in seconds) to make adjustments in the free of any known abnormalities. The establishment dosage of warfarin. Table 3 illustrates the fact that of the normal population mean must be accurate as the ISI of the thromboplastin reagent increases because it plays an important part in establishing (decreased sensitivity), the relative PT in seconds the equivalency of different laboratories’ INR values. An error of only one second can affect the INR as illustrated in Table . This normal PT mean is used Table 2
Effect of Mean Population PT on INR
ISI – International Sensitivity Index
For many years, after the introduction of the PT assay, clinical laboratories in the United States prepared their own thromboplastin. Those preparations varied markedly in their responsiveness to the anticoagulant effects of warfarin. Modern thromboplastin formulation, Although the INR is designed to make PT
based on recombinant DNA technology, is highly measurements from different laboratories
reproducible from lot to lot, with a clearly defined be comparable, experience has demonstrated
chemical matrix, and is highly sensitive to deficiencies that different instrument-reagent combinations
of Factors II, V, VII and X. Each batch of thromboplastin can occasionally affect the equivalency of
reagent is calibrated against the WHO standard and INR values.
assigned an ISI value. Spectra Laboratories is currently using recombinant thromboplastin for the performance of the PT assay. It is an ultra sensitive reagent with an ISI close to 1.3.
Summary
In summary, INR values are intended to assess stable patients on long-term oral
anticoagulant therapy. Caution should be used when comparing test results from different laboratories. In dialysis patients, residual heparin contamination of the sample can cause questionable results and require additional testing. Spectra Laboratories has instituted a heparin adsorption procedure to al eviate the problems of heparin interference. Call 800-433-3773 today for more information on how a renal-specific laboratory can References
Package Insert, INOTECH HEPARIN ADSORBENT PROCEDURE,
NCCLS. One Stage Prothrombin Time (PT) and Activated Partial Thromboplastin Time (APTT) Test; Approved Guideline. H47-A June 1996; 16:3.
Hirsh J, Poller L. The International Normalized Ratio: A guide to understanding and correcting its problems. Arch Intern Med. February 14, 1994; 154(3):8-8.
Lawrence J. Laboratory Monitoring of Anticoagulant Therapy: The Key Role Played by Preanalytical Variables. 1999 Fall/ Winter Hirsh J, Dalen JE, Deykin D, Poller L. Oral Anticoagulants: Mechanism of Vol 9 LabNotes BD Vacutainer Systems.
action, clinical effectiveness, and optimal therapeutic range. Chest. 199; 10 (suppl): 315-365.
Triplett D. International Normalized Ratio (INR): Use and Abuse. Coag Quarterly, MLA, 1993; :3.
Nichols W, Bowie W. Laboratory Medicine and Pathology. Mayo Clin Proc. 1993; 68:897-898.
Jensen R. Oral Anticoagulation and INR. Clinical Hemostasis Review. May 1999; 13:5.
NCCLS. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays. Brace L. Current Status of the International Normalized Ratio.
Approved Guideline. H1-A3, December 1998, 3rd Edition, Wayne,PA: NCCLS.
Laboratory Medicine. July 001; 7:3.
007 Fresenius Medical Care Holdings, Inc. All rights reserved.
48818 Kato Road • Fremont, CA 94538 • 800-433-3773 Spectra and are registered trademarks of 8 King Road • Rockleigh, NJ 07647 • 800-5-466 Fresenius Medical Care Holdings, Inc.
www.spectra-labs.com

Source: http://www.spectralaboratories.com/resources/pdf_scientific/ProThrom.pdf