Effect of thromboplastin and coagulometer interaction on the precision of the International Normalised Ratio

Adaptive force sonorheometry for assessment of whole blood coagulation

BACKGROUND

Viscoelastic diagnostics that monitor the hemostatic function of whole blood (WB), such as thromboelastography, have been developed with demonstrated clinical utility. By measuring the cumulative effects of all components of hemostasis, viscoelastic diagnostics have circumvented many of the challenges associated with more common tests of blood coagulation.

METHODS

We describe a new technology, called sonorheometry, that adaptively applies acoustic radiation force to assess coagulation function in WB. The repeatability (precision) of coagulation parameters was assessed using citrated WB samples. A reference range of coagulation parameters, along with corresponding measurements from prothrombin time (PT) and partial thromboplastin time (PTT), were obtained from WB samples of 20 healthy volunteers. In another study, sonorheometry monitored anticoagulation with heparin (0-5 IU/ml) and reversal from varied dosages of protamine (0-10 IU/ml) in heparinized WB (2 IU/ml).

RESULTS

Sonorheometry exhibited low CVs for parameters: clot initiation time (TC1), <7%; clot stabilization time (TC2), <6.5%; and clotting angle (theta), <3.5%. Good correlation was observed between clotting times, TC1 and TC2, and PTT (r=0.65 and 0.74 respectively; n=18). Linearity to heparin dosage was observed with average linearity r>0.98 for all coagulation parameters. We observed maximum reversal of heparin anticoagulation at protamine to heparin ratios of 1.4:1 from TC1 (P=0.6) and 1.2:1 from theta (P=0.55).

CONCLUSIONS

Sonorheometry is a non-contact method for precise assessment of WB coagulation.

Delivery of Optimized Anticoagulant Therapy: Consensus Statement from the Anticoagulation Forum

Objective:

To provide recommendations, policies, and procedures pertaining to the provision of optimized anticoagulation therapy designed to achieve desired clinical endpoints while minimizing the risk of anticoagulant-related adverse outcomes (principally bleeding and thrombosis).

Study Selection and Data Extraction:

Due to this document's scope, the medical literature was searched using a variety of strategies. When possible, recommendations are supported by available evidence; however, because this paper deals with processes and systems of care, high-quality evidence (eg, controlled trials) is unavailable. In these cases, recommendations represent the consensus opinion of all authors who constitute the Board of Directors of The Anticoagulation Forum, an organization dedicated to optimizing anticoagulation care. The Board is composed of physicians, pharmacists, and nurses with demonstrated expertise and significant collective experience in the management of patients receiving anticoagulation therapy.

Data Synthesis:

Recommendations for delivering optimized anticoagulation therapy were developed collaboratively by the authors and are summarized in 9 key areas: (I) Qualifications of Personnel, (II) Supervision, (III) Care Management and Coordination, (IV) Documentation. (V) Patient Education, (VI) Patient Selection and Assessment, (VII) Laboratory Monitoring, (VIII) Initiation and Stabilization of Warfarin Therapy, and (IX) Maintenance of Therapy. Recommendations are intended to inform the development of care systems containing elements with demonstrated benefit in improvement of anticoagulation therapy outcomes. Recommendations for delivering optimized anticoagulation therapy are intended to apply to all clinicians involved in the care of outpatients receiving anticoagulation therapy, regardless of the structure and setting in which that care is delivered.

Conclusions:

Anticoagulation therapy, although potentially life-saving, has inherent risks. Whether a patient is managed in a solo practice or a specialized anticoagulation management service, a systematic approach to the key elements outlined herein will reduce the likelihood of adverse events. The need for continued research to validate optimal practices for managing anticoagulation therapy is acknowledged.

Tibolone (Livial) enhances warfarin-induced anticoagulation in postmenopausal women

OBJECTIVE

To investigate the potential drug interaction between tibolone and warfarin in healthy postmenopausal women.

METHODS AND RESULTS

The study was designed as a double-blind, randomized, placebo-controlled, two-way crossover study in postmenopausal women. After stabilization of the International Normalized Ratio (INR; a standardized prothrombin time, PT) between 1.4 and 2.0 with warfarin, subjects were randomized to receive either tibolone (2.5mg/day) or placebo for 21 days. After a 7-day wash-out period (during which warfarin treatment was continued) the treatments were crossed over. Primary efficacy parameters were INR and coagulation Factors II, VII, VIIa and X (means of measurements at Days 18 and 20 and Days 46 and 48). Treatment with tibolone induced a statistically significant increase in INR (estimate of mean difference=0.40; P=0.002), and a statistically significant decrease in coagulation factors. Treatments were generally well tolerated and no clinically significant adverse events were observed.

CONCLUSIONS

Tibolone enhances warfarin-induced anticoagulation in postmenopausal women, as reflected by increases in INR and decreases in coagulation Factors II, VII, VIIa and X, compared to placebo. It is advisable to monitor for changes in coagulation status during (and after discontinuation of) simultaneous use of tibolone and warfarin.

Clinical utilization of the international normalized ratio (INR)

The prothrombin time (PT) is one of the most important laboratory tests to determine the functionality of the blood coagulation system. It is used in patient care to diagnose diseases of coagulation, assess the risk of bleeding in patients undergoing operative procedures, monitor patients being treated with oral anticoagulant (coumadin) therapy, and evaluate liver function. The PT is performed by measuring the clotting time of platelet-poor plasma after the addition of calcium and thromboplastin, a combination of tissue factor and phospholipid. Intra- and interlaboratory variation in the PT was a significant problem for clinical laboratories in the past, when crude extracts of rabbit brain or human placenta were the only source of thromboplastin. The international normalized ratio (INR), developed by the World Health Organization in the early 1980s, is designed to eliminate problems in oral anticoagulant therapy caused by variability in the sensitivity of different commercial sources and different lots of thromboplastin to blood coagulation factor VII. The INR is used worldwide by most laboratories performing oral anticoagulation monitoring, and is routinely incorporated into dosage planning for patients receiving warfarin. Although the recent availability of sensitive PT reagents prepared from recombinant human tissue factor (rHTF) and synthetic phospholipids eliminated many of the earlier problems associated with the use of crude thromboplastin preparations, local instrument variability in the INR still remains a problem. Presently, the use of plasma calibrants seems the best solution to this problem. Standardizing the point-of-care instruments for INR monitoring is another dilemma faced by the industry. Ultimately, new generations of anticoagulant drugs may eliminate the need for laboratory monitoring of anticoagulant therapy.

Accuracy and precision of the TAS analyser for near-patient INR testing by non-pathology staff in the community

AIMS

To assess the accuracy and precision of INR measurement by trained practice and district nursing staff using the Thrombolytic Assessment System (TAS) analyser.

METHODS

Seventeen nurses from four practices were trained to measure INR using the TAS analyser on citrated capillary blood samples. Quality control (QC) consisted of: daily internal QC using normal and abnormal commercial plasmas; monthly local external QC scheme using fresh citrated venous blood; and registration of all analysers in the NEQAS (national external quality assessment scheme) main users scheme.

RESULTS

Analysis of internal QC results demonstrated satisfactory interanalyser and intra-analyser precision with no evidence of analytical drift in any of the four practice analysers over an eight month period. Local and national external QC results confirmed the interanalyser precision but INR was underestimated by the TAS analysers compared with the CA 1000 using either Diagen rabbit brain thromboplastin or Innovin, and with other NEQAS users.

CONCLUSIONS

The TAS analyser has many features to commend it for use by nonpathology staff to determine INR. Local internal and external QC and entry into the NEQAS main users group are possible because the TAS analyses citrated plasma or blood. The TAS analyser underestimates INR when the geometric mean normal prothrombin time (GMNPT) is determined by conventional methods. A local correction factor can be introduced by adjusting the normal PT to give INR results comparable with the local laboratory. This is particularly desirable when INRs are measured using both near-patient and laboratory analytical systems on different occasions.