Coagulation Testing Guidelines

Introduction to PT/INR Testing

The International Society on Thrombosis and Haemostasis recommends using the INR scale for patients on vitamin K antagonists with no other superimposed coagulation defect, as it is only validated for this population 1, 2
The American Association for the Study of Liver Diseases suggests using a modified INR-liver for cirrhosis patients, as the INR scale has limitations for monitoring coagulation in patients with liver disease 1

The normal range for INR is 0.8-1.2, and the therapeutic range for warfarin is INR 2.0-3.0 for most indications, as recommended by the American College of Cardiology 2
An INR >4.0 is associated with a higher bleeding risk without additional therapeutic benefit, and the American Heart Association recommends careful monitoring in such cases 2

The American Society of Hematology recommends avoiding plasma transfusion based solely on mildly elevated INR (1.0-2.0) in non-bleeding patients not on anticoagulants, with a strength of evidence rated as high 2
Patients with liver disease, lupus anticoagulant, or those on direct oral anticoagulants (DOACs) require special consideration when interpreting PT/INR results, and alternative coagulation tests may be necessary, as recommended by the European Association for the Study of the Liver and Praxis Medical Insights 1, 2, 3

The INR is calculated using the formula: INR = (patient PT/normal PT)^ISI, where ISI is the International Sensitivity Index specific to the laboratory's thromboplastin reagent and instrument, as standardized by the International Organization for Standardization and recommended by the Clinical and Laboratory Standards Institute 1, 2
The American College of Cardiology recommends using PT/INR as the primary monitoring tool for patients on vitamin K antagonist therapy, with a target INR of 2.0-3.0 for most indications 4, 2

For liver disease, consider using INR-liver (modified-INR valid for cirrhosis) rather than standard INR-vka, as suggested by the Journal of Hepatology 1
For lupus anticoagulant, consider using diluted Russell's viper venom time (dRVVT), TTPA with confirmation test, or chromogenic factor X activity, according to Praxis Medical Insights 3
Thrombin generation assays may be more suitable for monitoring anticoagulation in special populations, though they are not available in every laboratory, as noted by the Journal of Hepatology 1
The 2023 WSES guidelines recommend performing multiple coagulation assays, including aPTT, TT, PT, INR, and anti-Xa levels, for a more complete picture of coagulation status 5

In patients with liver disease, evaluate other coagulation factors (VII, V, II) in addition to INR, as recommended by the Journal of Hepatology 1
In patients with lupus anticoagulant, consider using chromogenic factor X assay or thromboplastins less sensitive to lupus anticoagulant, and consider DOACs as an alternative to VKAs, according to Praxis Medical Insights 3
INR targets are poor predictors of bleeding in patients with liver disease and acute illness, as highlighted by the Journal of Thrombosis and Haemostasis 2