Monitoring Oral Anticoagulant Therapy with Vitamin K Antagonists

Croatian Society of Medical Biochemistry and Laboratory Medicine: National recommendations for blood collection, processing, performance and reporting of results for coagulation screening assays prothrombin time, activated partial thromboplastin time

“…Whenever it is possible ISI value specific for local thromboplastin/coagulometer combination should be used as it provides greater accuracy in INR over the use of a generic ISI (an ISI determined for a thromboplastin that is not coagulometer-specific but rather for a group of coagulometers). When ISI value for a specific thromboplastin/coagulometer combination is not available it is recommended to determine it locally ( 29 , 30 , 32 ). Briefly, this could be achieved by using a set of lyophilized plasma samples certified in terms of PT obtained by use of IRP and manual technique.…”

“…This method also increases the accuracy of INR determination and achieves better comparability of results between laboratories. For further details, readers are referred to references 29, 30 and 32.…”

“…Considering VKA therapy monitoring, the INR system for reporting PT results across different laboratories should be used exclusively, whereas in all other clinical indications INR should not be reported ( 30 ). Reporting results as PT-ratio cannot be used for reliable dose-adjustment for patients on VKA therapy as this could only be applicable to a particular brand of thromboplastin.…”

“…In general, INR values between 2.0 and 3.0 are the proposed therapeutic interval for the majority of clinical indications, although for mechanical heart valves the target INR range might be slightly higher (2.0-3.5) ( 30 , 33 ). INR values above the recommended therapeutic interval indicates increased bleeding risk, whereas values below the therapeutic interval are associated with increased risk of thrombosis.…”

“…It is important to note that INR value should always be reported as a numerical value up to a certain measurement limit, which is variable depending on the calibration curve obtained for a specific reagent lot. It is not perfect in terms of results comparability between laboratories but it reduces the variation and provides clinically useful results ( 29 , 30 ).…”