Anticoagulant added to blood for estimation of prothrombin time is:

There are many variables that can affect the PT/INR result outcome.In fact, preanalytical variables account for up to 64% of all errors in PT/INR testing.One of the most impoant factors is the anticoagulant used in drawing the blood specimen.The World Health Organization guidelines recommend the use of 3.2% buffered citrate.The evacuated tube must be completely filled (at least 90% full) to maintain the proper anticoagulant-to-blood ratio. It is best to avoid traumatic venipunctures to minimize the release of tissue factor, which can initiate coagulation.The blood sample must be centrifuged for sufficient time (10 minutes) to create platelet-poor plasma as the presence of platelets in the specimen can shoen clotting times. Specimens for FT testing may be stored at room temperature and will yield valid results for specimens stored up to 24 hours, provided that the collection tube remains unopened.