This is a multi-center study designed to evaluate the heparin insensitivity of the INRatio Prothrombin Time (PT) Monitoring System, utilizing an INRatio test strip additionally modified for low sample volume. The INRatio test strip is used for the quantitative determination of PT and International Normalized Ratio (INR) results in fingerstick blood from subjects on oral anticoagulation therapy (OAT) with warfarin. This study is designed to evaluate the accuracy of the modified INRatio test strip during heparin-warfarin bridge therapy with unfractionated heparin (UH) or low molecular weight heparin (LMWH), such as enoxaparin or dalteparin. These INR results will be compared to the INR results obtained on plasma from the same subjects as analyzed at a central laboratory with the heparin-insensitive reference method: the Sysmex CA-560 System. The levels of UH or LMWH in the plasma samples will be assessed using activated partial thromboplastin time (aPTT) and anti-factor-Xa assays respectively.

Inclusion Criteria:

• Adults (18 years of age or older).
• Willing and able to provide written informed consent and comply with study procedures.
• Receiving warfarin/heparin bridge therapy.
• Therapy with both warfarin and one of the heparins listed below must have been initiated for at least 12 hours before the first study specimens are obtained.
• UH (last known aPTT prior to enrollment (if available†) must confirm heparin has been administered)
• Dalteparin (last known anti-factor Xa prior to enrollment (if available†) must confirm heparin has been administered)
• Enoxaparin (last known anti-factor Xa prior to enrollment (if available†) must confirm heparin has been administered) † If a pre-study aPTT or anti-factor Xa is not available, subsequent testing of the study phlebotomy or a phlebotomy as a part of routine medical care taken before or during the time of the study phlebotomy must confirm the presence of heparin (unfractionated or low molecular weight) in order for that data point to be included in the analysis.

Exclusion Criteria

• Known or suspected hematocrit less than 25 or greater than 55%;
• Auto-immune disorders known to interfere with INR measurements, such as lupus or anti-phospholipid syndrome (APS)
• Already participated in this specific study

Patients undergoing heparin bridging onto OAT with warfarin.