The influence of antiphospholipid antibodies on INR values measured with the CoaguChek XS

Antiphospholipid syndrome (APS) is characterized by recurrent thrombosis or pregnancy complications in patients with persistent antiphospholipid antibodies. Patients with APS receive anticoagulant therapy with vitamin K antagonists (VKA) to prevent recurrent thrombosis. VKA treatment can be monitored with the international normalized ratio (INR), which is based on clotting tests. The optimal therapeutic window for VKA is an INR between 2.0 and 3.0. An INR < 2.0 is associated with an increased risk of thrombosis and an INR > 3.0 is associated with an increased risk of bleeding. Frequent monitoring and, if necessary, VKA dose adaptation, ensures patients receive adequate anticoagulation. Whilst the INR is routinely measured with clotting tests in a diagnostic laboratory, many patients monitor their own INR with Point Of Care (POC) devices. However, antiphospholipid antibodies can interfere with clotting reactions. Whereas INR reagents used in diagnostic laboratories are insensitive for interference by antiphospholipid antibodies, there are indications that reagents in POC devices are not, which could lead to false INR values and inadequate anticoagulation.
In the current study, we will investigate whether INR values in APS patients measured with the most commonly used POC device in the Netherlands (CoaguChek XS) are similar to the gold standard method used in the UMC Utrecht diagnostic laboratory; the Owren method based on a rabbit brain-derived thromboplastin.

We expect differences in INR values between the CoaguCHek and Coagulometer in lupus anticoagulant positive APS patients.

1x venepuncture and 1x finger stick per patient

Patients will endure 1 finger stick procedure and 1 venepuncture,