Determination of the International Sensitivity Index of a New Near-Patient Testing Device to Monitor Oral Anticoagulant Therapy

Vitamin K antagonists in heart disease: current status and perspectives (Section III). Position paper of the ESC Working Group on Thrombosis--Task Force on Anticoagulants in Heart Disease

Oral anticoagulants are a mainstay of cardiovascular therapy, and for over 60 years vitamin K antagonists (VKAs) were the only available agents for long-term use. VKAs interfere with the cyclic inter-conversion of vitamin K and its 2,3 epoxide, thus inhibiting γ-carboxylation of glutamate residues at the amino-termini of vitamin K-dependent proteins, including the coagulation factors (F) II (prothrombin), VII, IX and X, as well as of the anticoagulant proteins C, S and Z. The overall effect of such interference is a dose-dependent anticoagulant effect, which has been therapeutically exploited in heart disease since the early 1950s. In this position paper, we review the mechanisms of action, pharmacological properties and side effects of VKAs, which are used in the management of cardiovascular diseases, including coronary heart disease (where their use is limited), stroke prevention in atrial fibrillation, heart valves and/or chronic heart failure. Using an evidence-based approach, we describe the results of completed clinical trials, highlight areas of uncertainty, and recommend therapeutic options for specific disorders. Although VKAs are being increasingly replaced in most patients with non-valvular atrial fibrillation by the new oral anticoagulants, which target either thrombin or FXa, the VKAs remain the agents of choice for patients with atrial fibrillation in the setting of rheumatic valvular disease and for those with mechanical heart valves.

Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

The objective of this article is to summarize the published literature concerning the pharmacokinetics and pharmacodynamics of oral anticoagulant drugs that are currently available for clinical use and other aspects related to their management.

METHODS

We carried out a standard review of published articles focusing on the laboratory and clinical characteristics of the vitamin K antagonists; the direct thrombin inhibitor, dabigatran etexilate; and the direct factor Xa inhibitor, rivaroxaban

RESULTS

The antithrombotic effect of each oral anticoagulant drug, the interactions, and the monitoring of anticoagulation intensity are described in detail and discussed without providing specific recommendations. Moreover, we describe and discuss the clinical applications and optimal dosages of oral anticoagulant therapies, practical issues related to their initiation and monitoring, adverse events such as bleeding and other potential side effects, and available strategies for reversal.

CONCLUSIONS

There is a large amount of evidence on laboratory and clinical characteristics of vitamin K antagonists. A growing body of evidence is becoming available on the first new oral anticoagulant drugs available for clinical use, dabigatran and rivaroxaban.

Pharmacology and management of the vitamin K antagonists: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This article concerning the pharmacokinetics and pharmacodynamics of vitamin K antagonists (VKAs) is part of the American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). It describes the antithrombotic effect of the VKAs, the monitoring of anticoagulation intensity, and the clinical applications of VKA therapy and provides specific management recommendations. Grade 1 recommendations are strong and indicate that the benefits do or do not outweigh the risks, burdens, and costs. Grade 2 recommendations suggest that the individual patient's values may lead to different choices. (For a full understanding of the grading, see the "Grades of Recommendation" chapter by Guyatt et al, CHEST 2008; 133:123S-131S.) Among the key recommendations in this article are the following: for dosing of VKAs, we recommend the initiation of oral anticoagulation therapy, with doses between 5 mg and 10 mg for the first 1 or 2 days for most individuals, with subsequent dosing based on the international normalized ratio (INR) response (Grade 1B); we suggest against pharmacogenetic-based dosing until randomized data indicate that it is beneficial (Grade 2C); and in elderly and other patient subgroups who are debilitated or malnourished, we recommend a starting dose of < or = 5 mg (Grade 1C). The article also includes several specific recommendations for the management of patients with nontherapeutic INRs, with INRs above the therapeutic range, and with bleeding whether the INR is therapeutic or elevated. For the use of vitamin K to reverse a mildly elevated INR, we recommend oral rather than subcutaneous administration (Grade 1A). For patients with life-threatening bleeding or intracranial hemorrhage, we recommend the use of prothrombin complex concentrates or recombinant factor VIIa to immediately reverse the INR (Grade 1C). For most patients who have a lupus inhibitor, we recommend a therapeutic target INR of 2.5 (range, 2.0 to 3.0) [Grade 1A]. We recommend that physicians who manage oral anticoagulation therapy do so in a systematic and coordinated fashion, incorporating patient education, systematic INR testing, tracking, follow-up, and good patient communication of results and dose adjustments [Grade 1B]. In patients who are suitably selected and trained, patient self-testing or patient self-management of dosing are effective alternative treatment models that result in improved quality of anticoagulation management, with greater time in the therapeutic range and fewer adverse events. Patient self-monitoring or self-management, however, is a choice made by patients and physicians that depends on many factors. We suggest that such therapeutic management be implemented where suitable (Grade 2B).

Guidelines for implementation of patient self-testing and patient self-management of oral anticoagulation. International consensus guidelines prepared by International Self-Monitoring Association for Oral Anticoagulation

AIMS

This document provides health care professionals involved in initiating and monitoring oral anticoagulation therapy with guidelines for the provision of safe and effective patient self-testing/patient self-management of oral anticoagulation.

METHODS AND RESULTS

The consensus group has critically reviewed the literature and compared the results of usual care (UC) vs. anticoagulation clinic and patient self-management/patient self-testing (PSM/PST). The education and training of patients for self-monitoring are described, together with the suitability of patients, the effect on quality of life and cost-effectiveness. The consensus agrees that patient self-testing and patient self-management are effective methods of monitoring oral anticoagulation therapy, providing outcomes at least as good as, and possibly better than, those achieved with an anticoagulation clinic. All patients must be appropriately selected and trained. Currently available self-testing/self-management devices give INR results which are comparable with those obtained in laboratory testing. The most frequent testing frequency is weekly but lower frequency of testing can be justified based on institutional or patient conditions.

CONCLUSIONS

The consensus agrees that there are several points in favour of PST/PSM, for example, a higher degree of medical safety, increased patient education, improved response to changes in lifestyle, increased independence for the patient and improved quality of life.

The pharmacology and management of the vitamin K antagonists: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy

This article concerning the pharmacokinetics and pharmacodynamics of vitamin K antagonists (VKAs) is part of the Seventh American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. The article describes the antithrombotic effect of VKAs, the monitoring of anticoagulation intensity, the clinical applications of VKA therapy, and the optimal therapeutic range of VKAs, and provides specific management recommendations. Grade 1 recommendations are strong, and indicate that the benefits do, or do not, outweigh the risks, burdens, and costs. Grade 2 suggests that individual patient's values may lead to different choices (for a full understanding of the grading see Guyatt et al, CHEST 2004; 126:179S-187S). Among the key recommendations in this article are the following: for dosing of VKAs, we suggest the initiation of oral anticoagulation therapy with doses between 5 and 10 mg for the first 1 or 2 days for most individuals, with subsequent dosing based on the international normalized ratio (INR) response (Grade 2B). In the elderly and in other patient subgroups with an elevated bleeding risk, we suggest a starting dose at < or = 5 mg (Grade 2C). We recommend basing subsequent doses after the initial two or three doses on the results of INR monitoring (Grade 1C). The article also includes several specific recommendations for the management of patients with INRs above the therapeutic range and for patients requiring invasive procedures. For example, in patients with mild to moderately elevated INRs without major bleeding, we suggest that when vitamin K is to be given it be administered orally rather than subcutaneously (Grade 1A). For the management of patients with a low risk of thromboembolism, we suggest stopping warfarin therapy approximately 4 days before they undergo surgery (Grade 2C). For patients with a high risk of thromboembolism, we suggest stopping warfarin therapy approximately 4 days before surgery, to allow the INR to return to normal, and beginning therapy with full-dose unfractionated heparin or full-dose low-molecular-weight heparin as the INR falls (Grade 2C). In patients undergoing dental procedures, we suggest the use of tranexamic acid mouthwash (Grade 2B) or epsilon amino caproic acid mouthwash without interrupting anticoagulant therapy (Grade 2B) if there is a concern for local bleeding. For most patients who have a lupus inhibitor, we suggest a therapeutic target INR of 2.5 (range, 2.0 to 3.0) [Grade 2B]. In patients with recurrent thromboembolic events with a therapeutic INR or other additional risk factors, we suggest a target INR of 3.0 (range, 2.5 to 3.5) [Grade 2C]. As models of anticoagulation monitoring and management, we recommend that clinicians incorporate patient education, systematic INR testing, tracking, and follow-up, and good communication with patients concerning results and dosing decisions (Grade 1C+).

Home prothrombin time monitoring: a literature analysis

The anticoagulant activity of warfarin sodium is monitored by the prothrombin time (PT) using the international normalized ratio (INR). Standard oral anticoagulant therapy monitoring requires frequent patient visits to physicians' offices and/or laboratories to optimize warfarin dosage. Home PT monitoring by patients can increase testing frequency and may thus decrease complications associated with oral anticoagulant therapy. Clinical studies suggest that home PT monitoring is more effective than uncoordinated management and is as effective as care through specialized anticoagulation clinics for keeping INRs within a therapeutic range. There are accurate and reliable instruments available, but paramount to the success of home PT monitoring is sound patient selection, appropriate patient training, and consistent quality control.

Quality assurance program for whole blood prothrombin time–international normalized ratio point-of-care monitors used for patient self-testing to control oral anticoagulation

Whole blood coagulation monitors are increasingly used for patient self-testing to control oral anticoagulation, but there are no comprehensive quality assurance (QA) programs to check their performance. We report on the experience with one of such programs applied in a field study where patients on prothrombin time (PT)-international normalized ratio (INR) self-testing were asked to bring their monitors to the anticoagulation clinic for checking. PT-INR testing was performed three times over 3 months with 14 patient's monitors and test strips on three recalcified QA plasmas by an experienced laboratory operator. Each patient was also asked to perform PT-INR self-testing (his/her own capillary blood) which was then compared to the laboratory PT-INR (plasma). Overall, the comparison between the observed and the consensus PT-INR on QA plasmas was acceptable with the majority of measurements lying within +/-15% or 20% of the consensus values. The comparison between the PT-INR self-testing and the laboratory method was also acceptable: overall, there was no statistical significant difference between the mean PT-INR values and the majority of paired measurements were less than 15% or 20% apart. In conclusion, our results show that the proposed QA scheme is feasible and may be implemented on a larger scale. Monitors should be recalled periodically to the clinic where they have been prescribed to the patient. During each visit, the clinic may check the monitors and patient self-testing performance as described. Such comprehensive QA system would make monitoring of oral anticoagulant treatment by self-testing safer and more effective.

European Concerted Action on Anticoagulation (ECAA): an assessment of a method for ISI calibration of two whole blood point-of-care PT monitor systems based on lyophilized plasmas using whole blood equivalent PT

Previously, the attempt to simplify the International Sensitivity Index (ISI) calibration of the CoaguChek Mini whole blood point-of-care test prothrombin time (PT) monitor system was successful using lyophilized plasmas from coumarin-treated patients but not with lyophilized artificially depleted plasmas. With the TAS PT-NC monitor system, both types of plasma failed to provide reliable calibrations. The present study assesses a procedure for the ISI calibration of a TAS PT-NC and CoaguChek Mini whole blood point-of-care test PT monitor systems using lyophilized plasmas. Using lyophilized artificially depleted and coumarin plasma calibrations, we have evaluated a correction for the monitor displayed PT. This was based on a 'line of equivalence' derived from the relationship between whole blood and fresh plasma PT with both types of monitor system. With the TAS PT-NC, the use of this 'line of equivalence' resulted in reliable ISI with both lyophilized coumarin and artificially depleted plasmas. There was no significant difference between mean monitor and mean reference International Normalized Ratio (INR) with the artificially depleted plasmas. With the lyophilized coumarin plasma calibrations there was only a small INR difference. Correction with the 'line of equivalence' therefore facilitates calibration of the TAS PT-NC with lyophilized plasmas. With the CoaguChek Mini, the correction based on the 'line of equivalence' did not improve results but was not required with this system.

European Concerted Action on Anticoagulation. Evaluation of a Method for International Sensitivity Index Calibration of Two Point-of-Care Prothrombin Time (PT) Monitoring Systems (CoaguChek Mini and TAS PT-NC) with Fresh Plasmas Based on Whole-Blood Equivalent PT

Background: The International Sensitivity Index (ISI) calibration of whole-blood prothrombin time (PT) monitors for point-of-care testing (POCT) described by Tripodi et al. (Thromb Haemost 1993;70:921–4) has been shown to be dependable but is too complex and demanding. The use of plasma would simplify calibration of whole-blood POCT PT monitors, but important differences may exist between the ISI for whole blood and plasma calibrations.

Methods: In a 10-center calibration study of two POCT whole-blood monitoring systems (CoaguChek Mini and TAS PT-NC), we characterized the relationship between the log PT for whole blood and fresh plasma with use of single lots of test strips/cards. This relationship (linear) was used to correct the difference between the whole-blood and plasma ISI. The reliability of the correction with different lots of test strips/cards was assessed at three centers. The linear relationship was used to correct the difference in the whole-blood and plasma ISI with four other lots of TAS PT-NC cards and with two additional lots of CoaguChek Mini test strips.

Results: The correction decreased the ISI difference from 13.3% to 0.9% for the TAS PT-NC and from 5.7% to 0.6% for the CoaguChek Mini. In assessments at three centers, which included different lots of test strips/cards, the mean ISI difference was markedly decreased with the TAS PT-NC but not with the CoaguChek Mini, for which the mean ISI difference increased slightly.

Conclusions: The proposed correction resolves the discrepancy between whole-blood and fresh plasma ISI calibrations with TAS PT-NC test cards. The CoaguChek Mini systems could be calibrated without this correction.

Minimum numbers of fresh whole blood and plasma samples from patients and healthy subjects for ISI calibration of CoaguChek and RapidPointCoag monitors

The international sensitivity index (ISI) calibration of point-of-care-test (POCT) prothrombin time (PT) whole blood monitors is complex, requiring manual PT testing of 60 patients' and 20 healthy subjects' plasma samples. The possibility of reducing these numbers was studied by a Monte Carlo Bootstrap study for 2 POCT PT systems. For reduced sample numbers, this consisted of 50,000 calibrations using whole blood and plasma samples tested on the monitors with manual PT testing of plasma samples from the same blood donations. There was little effect on mean ISI by reduction of sample numbers to a total of 7, but there was progressively less certainty regarding the reliability of the calibration. Precision of the calibrations and international normalized ratio deviation were not affected markedly by reducing numbers to half As ISI calibration with the 2 POCT systems was less precise than conventional manual testing, for maximum confidence, reduction of numbers is not advised.

The quality of oral anticoagulation before, during and after a period of patient self-management

The study analyzes the quality of anticoagulation during a 3-year follow-up on patients who were treated by an anticoagulation clinic (ACS) for 1 year (Phase I), performed weekly self-management of anticoagulation (PSM) after a specific training for another year (Phase II) and finally returned to be treated by the anticoagulation clinic (ACS) for a third year (Phase III). The mean fraction of INR values within therapeutic target range was higher in Phase II (0.69 +/- 0, 11) compared to Phases I (0.40 +/- 0.20) and III (0.56 +/- 0.18; p < 0.05). Time spent in therapeutic target range was higher in Phase II (0.70 +/- 0.10) compared to Phases I (0.43 +/- 0.25) and III (0.60 +/- 0.17; p < 0.05). Mean square deviation from target value was lower in Phase II (0.39 +/- 0.17) compared to Phases I (0.81 +/- 0.44) and III (0.64 +/- 0.39, p = 0.05). Thus, the quality of anticoagulation during Phase II (PSM) was significantly better compared to Phases I (ACS) and III (ACS) in all endpoints tested. This shows that the quality of oral anticoagulation deteriorates again if patient self-management is stopped and patients return to conventional treatment. Furthermore, the quality of anticoagulation was better in Phase III (post-PSM) compared with Phase I (pre-PSM) although the type of treatment was identical in both phases (ACS). This suggests that the increased patient empowerment and enhanced compliance acquired during PSM (Phase II) might have a positive impact on the quality of anticoagulation, even when patients return to the conventional treatment (ACS).

European Concerted Action on Anticoagulation (ECAA): multicentre international sensitivity index calibration of two types of point-of-care prothrombin time monitor systems

A multicentre modified World Health Organization (WHO)-type international sensitivity index (ISI) calibration has been performed at 10 European Concerted Action on Anticoagulation (ECAA) national laboratories using non-citrated whole-blood on two point-of-care test (POCT) prothrombin time (PT) monitor systems, CoaguChek Mini and TAS PT-NC, using single lots of test cards/strips. The relevant species (human and rabbit) WHO international reference preparations (IRPs) were tested with the manual PT technique on citrated plasma from the same blood donations. The ISI was calculated from the slope of the orthogonal regression line relating log PT (POCT) to log PT (IRP). The mean ISI of the CoaguChek Mini system was 1.75 and 1.13 with the prothrombin time non-citrated Thrombolytic Assessment System (TAS PT-NC). With the CoaguChek Mini system, seven out of 10 calibrations exceeded the current 3% WHO recommended limit for the coefficient of variation (CV) of the slope with conventional PT testing, whereas with the TAS PT-NC system, it was eight out of 10. All the POCT calibrations had a CV of the slope <5%. It is suggested that this level of precision be adopted as the limit of acceptability of calibration of these monitor systems. In these circumstances, the modified WHO-type ISI calibration appeared to be satisfactory for the POCT whole-blood monitors.

European Concerted Action on Anticoagulation. Use of Plasma Samples to Derive International Sensitivity Index for Whole-Blood Prothrombin Time Monitors

Background: To simplify International Sensitivity Index (ISI) calibration, the possibility of substituting fresh plasma for fresh whole-blood samples with point-of-care testing (POCT) whole-blood monitors was investigated in a three-center study of three different POCT systems.

Methods: A modified full WHO calibration procedure based on 20 healthy controls and 60 coumarin-treated patients was performed on three monitoring systems with whole-blood and plasma samples against plasma tested using the European Concerted Action on Anticoagulation (ECAA) rabbit reference plain thromboplastin and the manual prothrombin time (PT) method.

Results: With one of the three systems, the mean ISI was 1.51 for whole blood and 1.49 for plasma; with the second system, the mean ISI was 1.08 for both whole blood and plasma. With the third system, however, the difference between the mean ISI for whole blood and that for plasma was greater (1.15 and 1.01, respectively). Overall, the precision of the calibrations was less than with traditional manual plasma PT testing.

Conclusions: Provided that an appropriate calcium chloride concentration is used, the plasma PT results can be used for accurate ISI calibration of two of these three whole-blood POCT systems. Precision criteria need to be modified for POCT monitors.

The oral anticoagulant saga: past, present, and future

Oral anticoagulation originated with the discovery of the harmful agent causing "sweet clover disease" in cattle in North America in the 1920s. The causative agent dicoumarol was isolated in Link's laboratory in 1940. A range of related compounds was then synthesized, the most popular of which proved to be warfarin. Oral anticoagulant administration posed problems of individual variation in response to these drugs and the need for regular laboratory monitoring by prothrombin time (PT). Monitoring problems arose from the introduction in the 1950s of some poorly responsive commercial tissue extracts for use as tissue extract thromboplastin reagent in the PT. More oral anticoagulant drug was then needed to prolong the test to the required therapeutic targets, with a resultant increase in bleeding. It was not until 1983 that the problem was resolved and it was shown that the less intense UK-type regimen was just as effective as the higher North American type dosage in the prevention of venous thrombosis but caused much less bleeding. This study led to the widespread adoption of the "low-dose warfarin" regimen that, combined with the World Health Organization PT standardization scheme using the international normalized ratio (INR), has led to improved effectiveness and safety of oral anticoagulation. This has permitted increased administration of warfarin in a widening spectrum of clinical disorders. The last remaining problem is the limited success of doctors in achieving the therapeutic INR targets, which may be improved by computer-assisted dosage.

Oral anticoagulation self-management and management by a specialist anticoagulation clinic: a randomised cross-over comparison

BACKGROUND

Vitamin K antagonist treatment is effective for prevention and treatment of thromboembolic events but frequent laboratory control and dose-adjustment are essential. Small portable devices have enabled patient self-monitoring of anticoagulation and self-adjustment of the dose. We compared this self-management of oral anticoagulant therapy with conventional management by a specialist anticoagulation clinic in a randomised cross-over study.

METHODS

50 patients on long-term oral anticoagulant treatment were included in a randomised controlled crossover study. Patients were self-managed or were managed by the anticoagulation clinic for a period of 3 months. After this period the alternative strategy was followed for each patient. Prothrombin time (expressed as international normalised ratio [INR]) were measured at intervals of 1-2 weeks in both periods without knowledge of type of management. The primary endpoint was the number of measurements within the therapeutic range (therapeutic target value +/-50.5 INR units).

FINDINGS

There was no significant difference in the overall quality of control of anticoagulation between the two study periods. Patients were for 55% and for 49% of the treatment period within a range of +/-0.5 from the therapeutic target INR during self-management and anticoagulation clinic management, respectively (p=0.06). The proportion of patients who spent most time in the therapeutic target range was larger during self-management than during anticoagulation clinic-guided management. The odds ratio for a better control of anticoagulation (defined as the period of time in the therapeutic target range) during self-management compared with anticoagulation clinic-guided management was 4.6 (95% CI 2.1-10.2). A patient-satisfaction assessment showed superiority of self-management over conventional care.

INTERPRETATION

Self-management of INR in the population in this study is feasible and appears to result in control of anticoagulation that is at least equivalent to management by a specialist anticoagulation clinic. It is also better appreciated by patients. Larger studies are required to assess the effect of this novel management strategy on the incidence of thromboembolic or bleeding complications.

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.