From the Guidelines
APTT (Activated Partial Thromboplastin Time) is a crucial blood test that measures the intrinsic and common pathways of the coagulation cascade, with normal values ranging from 25-35 seconds, and is commonly used to monitor heparin therapy and screen for clotting disorders, whereas PT (Prothrombin Time) and INR (International Normalized Ratio) assess the extrinsic and common pathways, with INR being a standardized measure across laboratories. The APTT test is essential in evaluating patients on anticoagulant therapy, such as heparin, and those with suspected clotting disorders, as it helps identify potential bleeding risks or thrombotic events 1. In the context of trauma patients, APTT and PT/INR are vital in assessing coagulopathy, particularly in those on direct oral anticoagulants (DOACs) like dabigatran, rivaroxaban, apixaban, and edoxaban, as these medications can affect the results of these tests, making interpretation challenging 1. Some key points to consider when interpreting APTT and PT/INR results include:
- APTT is more sensitive to dabigatran, a direct thrombin inhibitor, whereas PT is more sensitive to rivaroxaban, apixaban, and edoxaban, which are direct FXa inhibitors 1
- A normal APTT or PT/INR does not rule out the presence of DOACs, and viscoelastic coagulation tests like TEG and ROTEM may be useful in detecting coagulopathy associated with DOACs 1
- The results of APTT and PT/INR should be interpreted in the context of the patient's clinical presentation, medication history, and laboratory reference ranges, as the sensitivity and specificity of these tests can vary depending on the specific reagents and instruments used 1. It is essential to consider the limitations and variability of APTT and PT/INR when interpreting results, particularly in trauma patients on anticoagulant therapy, and to use viscoelastic coagulation tests and other diagnostic tools as needed to guide clinical decision-making 1.
From the FDA Drug Label
Warfarin sodium tablets may increase the activated partial thromboplastin time (aPTT) test, even in the absence of heparin A severe elevation (>50 seconds) in activated partial thromboplastin time (aPTT) with a PT/INR in the desired range has been identified as an indication of increased risk of postoperative hemorrhage.
The aPTT (activated partial thromboplastin time) is a test that measures the time it takes for blood to clot. On the PT (Prothrombin Time) and INR (International Normalized Ratio), an elevated aPTT can indicate an increased risk of bleeding, even if the PT/INR is within the desired range. This is relevant when using warfarin, as it can affect aPTT results, even in the absence of heparin 2.
From the Research
Understanding APTT, PT, and INR
- APTT stands for Activated Partial Thromboplastin Time, which is a test used to evaluate the intrinsic and common coagulation pathways 3.
- PT stands for Prothrombin Time, which is a test used to evaluate the extrinsic and common coagulation pathways 4.
- INR stands for International Normalized Ratio, which is a calculation based on PT and is used to monitor patients on warfarin therapy 3.
Relationship between APTT, PT, and INR
- Warfarin can affect APTT, and for each increase of 1.0 in INR, APTT increases by 16 seconds 3.
- The effects of warfarin and heparin on APTT are additive 3.
- APTT can be influenced by heparin, and the therapeutic goal is to keep APTT within a desirable range 4.
- PT is used as a guide for adjusting the coumadin dose during an overlap in the periods of coumadin and heparin administration 4.
Clinical Implications
- APTT is used to adjust heparin dosage, but warfarin can confound APTT results [(3,5)].
- Abnormal coagulation test results at pre-treatment baseline can provide an important clue regarding the risk of subsequent anticoagulant failure due to APTT or INR confounding 5.
- Alternative anticoagulants, such as fondaparinux or danaparoid, may be chosen to avoid APTT or INR confounding 5.