Precision and accuracy of CoaguChek S and XS monitors: The need for external quality assessment

Abstract

Thromb Haemost 2009; 101: 419–421 Greater recognition of the value of oral anticoagulation in a widening range of clinical disorders including atrial fibrillation (AF), has led to increasingly heavy demands for oral anticoagulant control worldwide, putting a great strain on the existing facilities in hospitals and community clinics. Anticoagulation to be safe and effective must conform to defined therapeutic intervals expressed as international normalised ratios (INR). INR less than 2.0 are generally ineffective in prevention of thrombosis whereas INR greater than 4.5 dramatically increase the risk of bleeding. To meet demands and ease the burden on existing facilities, a range of elegant, hand-held, easy to use, prothrombin time monitors designed for patient self-testing and to lesser extent patient self-management has been developed. By far the most popular and widely used is the CoaguChek monitor. Several hundred thousand are reported to be in use with finger-prick specimens in Germany alone, by patients engaged in self-monitoring. The CoaguChek is also stated to be employed by 150,000 patients in self-testing/self-dosage (1). It is important therefore that the CoaguChek instruments provide accurate INR results. The earlier models, CoaguChek and CoaguChek S, which have been in widespread clinical use and employed in clinical trials of patients’ self-testing/self-management for a number of years, are now being replaced by the CoaguChek XS. A United Kingdom Health Technology Assessment (2) in 2007 reported that in 16 selected randomised and eight non-randomised trials, patient self-monitoring of oral anticoagulation therapy was more effective than poor-quality usual care provided by family doctors and as effective as good-quality specialised anticoagulation clinics in maintaining the quality of anticoagulation therapy. There was, however, no significant reduction of major bleeding events between patient self-monitoring and usual care controls. Pooled analyses of results found that compared with primary care or anticoagulation control clinics, self-monitoring was associated with significantly fewer thromboembolic events. The conclusion was that for selected and successfully trained patients, self-monitoring is effective and safe for longterm oral anticoagulation. In general, although patient self-management is unlikely to be more cost-effective than the current specialised anticoagulation clinics in the UK, more self-monitoring may enhance the quality of life for some patients who are frequently away from home, in employment or education, or who find it difficult to travel to clinics. One of the important advantages of patient self-management is that anticoagulation control can be more frequent when tested by self-monitoring (every 1–2 weeks) than by usual care provided by family doctors or hospital clinics (every 2–4 weeks). Longer intervals up to 10 weeks are regular practice in some hospital clinics. The recent American College of Chest Physicians (ACCP) Guidelines (3) recommend a maximum four week interval between tests but this is frequently exceeded in routine practice. More frequent testing is easier with CoaguChek self-testing than with other control methods and generally improves the percentage of time in target INR range. Results from controlled trials were summarised in the UK Assessment by stating that patient self-management is better than poor-quality anticoagulation control provided by family doctors, particularly in the prevention of inadequate anticoagulation (proportion of time INR spent below the target therapeutic range). This was on average 19% in patients using self-monitoring compared with 33% in patients managed by family doctors. Overall CoaguChek patient self-management was as effective as the usual care of specialised anticoagulant clinics. Patient selfmanagement is unlikely, however, to be more cost-effective than the current high-quality care provided by specialised anticoagulation clinics. In the current issue of Thrombosis and Haemostasis a team of Danish investigators, Christensen and colleagues, report a prospective study evaluating the accuracy and precision of a single CoaguChek S instrument and a single instrument of the newly introduced CoaguChek XS type, both tested in a group of patients on warfarin together with the local laboratory prothrombin time method (4). A total of 564 venous blood samples was obtained from 24 patients and tested weekly in parallel over a