This review will highlight the similarities and differences between the PT/INR vs. aPTT test for exams!
The PT/INR and aPTT are helpful tests for assessing the coagulation process. As the nurse you want to know their normal ranges, what each test assesses, how the tests are used to monitor anticoagulation therapy, and the nurse’s role.
Don’t forget to take the quiz on PT/INR vs. aPTT after reviewing this material.
PT/INR vs. aPTT Lecture
Prothrombin/INR vs. aPTT Blood Test Review
First, let’s talk about the similarities between the PT/INR and aPTT:
- Tell how fast the blood clots
- Assess pathways of the coagulation cascade (however, assesses different pathways)
- Measured in seconds
- Collected from a basic blood draw from the vein
- Assesses for bleeding disorders and helps monitor certain types of anticoagulation therapy (ex: Heparin or Warfarin)
- Sometimes are ordered together
Let’s talk a little more in depth about the coagulation cascade:
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.
Now, let’s talk about the differences between the PT/INR and aPTT:
PT/INR
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 take the anticoagulant Warfarin.
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).
The PT/INR tells us how well the extrinsic and common pathways work together. This will tell us how fast prothrombin turns in thrombin.
- Normal PT level is 10-12 seconds (approximate)
- High results mean the patient has a prolonged time forming a clot:
- 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 in labs 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
aPTT
aPTT stands for activated partial thromboplastin time.
The aPTT assesses the INTRINSIC and COMMON pathways. Therefore, it checks clotting factors: I, II, V, X (clotting factors of the common pathways), and XII, XI, IX, VIII (clotting factors of the intrinsic pathway).
- The normal range measurement varies among labs because they use different testing agents.
- Normal range for an aPTT: 30-40 seconds (approximate)… results higher than this means the patient has a prolonged time forming a clot.
- aPTT vs PTT? It measures the same thing, but an activator agent is added to the aPTT to speed up the clotting time, which makes the results have a smaller range compared to the PTT.
- Normal range for PTT: 60-70 seconds (approximate)***NOTE: this time varies among sources and institutions. Some sources say a normal PTT is 25 to 35 seconds (source: https://my.clevelandclinic.org/health/diagnostics/25101-partial-thromboplastin-time and https://www.ncbi.nlm.nih.gov/books/NBK507772/
- The aPTT is measured for patients on Heparin because it more sensitive.
- Normal range for PTT: 60-70 seconds (approximate)***NOTE: this time varies among sources and institutions. Some sources say a normal PTT is 25 to 35 seconds (source: https://my.clevelandclinic.org/health/diagnostics/25101-partial-thromboplastin-time and https://www.ncbi.nlm.nih.gov/books/NBK507772/
- Heparin therapeutic goal is 1.5 – 2.5 times the normal value range.
You may be interested in Labs to Know for NCLEX Review.
References:
- Food and Drug administration. (2017). Heparin Sodium, for intravenous use Ebook]. Retrieved from https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/017029s140lbl.pdf
- Food and Drug administration. (2011). Warfarin sodium tablets Label Ebook]. Retrieved from https://www.accessdata.fda.gov/drugsatfda_docs/label/2011/009218s107lbl.pdf