This review will highlight the main concepts you need to know about the PT/INR test for exams!
The PT/INR is a helpful test for assessing the coagulation process. As the nurse you want to know the normal range, what the test assesses, how it’s used to monitor Warfarin therapy, and the nurse’s role.
Don’t forget to take the quiz on PT/INR vs. aPTT after reviewing this material.
PT/INR Lecture
Prothrombin/INR Blood Test Review
What’s a PT/INR?
A PT/INR is a blood test used to see how fast the blood clots. PT stands for prothrombin time and INR stands for international normalized ratio.
The INR is calculated from the PT level and is used to monitor patients who are taking the anticoagulant Warfarin.
To obtain the PT/INR, blood will be withdrawn from a vein with a needle, collected in a special vial, and sent to the lab. Then the lab will report the result back to you.
The PT/INR is helpful in detecting bleeding disorders because it assesses how well the coagulation cascade works. Also, it is sometimes ordered with an aPTT (which we will talk about in the next review).
The coagulation cascade is a process made up of 3 pathways that leads to the formation of a clot. This is beneficial when injury has occurred that could result in blood loss from the vascular system.
For example, when injury happens (whether it’s an external or internal injury), platelets arrive to the scene and this causes clotting factors to be activated in a specific order from specific pathways of the coagulation cascade. The end result will be that a clot forms and this will help prevent excessive bleeding.
The coagulation cascade includes 3 pathways that lead to the formation of a clot:
- extrinsic pathway: it is activated when there is external injury that results in blood loss to the vascular system.
- intrinsic pathway: it is activated when there is internal injury within the vascular system.
- common pathway: this is where the extrinsic and intrinsic pathway meet to form a clot.
*Each pathway has specific clotting factors that work to form a clot. There are approximately 13 clotting factors.
The PT/INR assesses the EXTRINSIC and COMMON pathways. Therefore, it checks clotting factors: I, II, V, X (clotting factors of the common pathways), and factor VII (clotting factor of the extrinsic pathway).
Therefore, the PT/INR tells us how well the extrinsic and common pathways work together. This will tell us how fast prothrombin turns in thrombin.
What’s prothrombin? It’s a clotting factor (specifically clotting factor II) and most clotting factors are made in the liver and depend on vitamin K. Therefore, this clotting factor plays a huge role in helping the body form a clot.
Prothrombin turns into thrombin with the assistance of clotting factor V. When thrombin is on board it is responsible for turning fibrinogen to fibrin. Fibrin is one of the main ingredients for clot formation because it’s a strong mesh like substance that helps create a plug to help prevent excessive bleeding (the mesh causes platelets and other blood cells to stick to it).
Key Concepts to Remember about the PT/INR
- The PT test tells us about how well the extrinsic and common pathway work together, which helps us know how fast prothrombin can turn into thrombin.
- It’s measured in seconds.
- The normal range measurement varies among labs because they use different testing agents.
- Normal PT level is approximately 10-12 seconds.
- High results mean the patient has a prolonged time forming a clot:
- Causes: clotting disorders, vitamin k deficiency, types of cancers, liver disease, or the patient is taking an anticoagulant
- High results mean the patient has a prolonged time forming a clot:
Because the PT range varies and there needs to be a standard way of reporting this level, especially for the patient taking Warfarin (because this medication’s dosage is calculated based on this result), the International Normalized Ratio (INR) is used. The INR is calculated from prothrombin time.
- Normal INR is less than 1.1
- Warfarin: to be therapeutic 2-3
- <2 medication not effective at preventing clots
- >3 risk of bleeding
You may be interested in Labs to Know for NCLEX Review.
References:
- Food and Drug administration. (2011). Warfarin sodium tablets Label Ebook]. Retrieved from https://www.accessdata.fda.gov/drugsatfda_docs/label/2011/009218s107lbl.pdf