Test library
Prothrombin Time and INR Test
This blood test measures prothrombin time and calculates the INR, a standardized clotting measure. It is used to evaluate coagulation and monitor anticoagulant therapy.
Order Prothrombin Test: Prothrombin Time PT with INR
Order online and visit a local collection site, or choose an at-home kit where available. No referral and no insurance required.
Order Prothrombin Test: Prothrombin Time PT with INR- Typical turnaround
- 1-3 business days
- Starting price
- From $29
- Results
- Confidential, online
Quick answer
The Prothrombin Time with INR Test measures how long it takes blood plasma to clot through the extrinsic clotting pathway and converts that time into a standardized value called the INR. Prothrombin time alone can vary between laboratories, while the INR adjusts for the testing reagent so results can be compared. The test is used to evaluate clotting function and to monitor certain anticoagulant therapies when prescribed by a healthcare provider.
What this test measures
Clotting is a stepwise process involving multiple proteins called factors. Prothrombin time reflects the activity of several of these factors, including factors II, V, VII, and X, along with fibrinogen. The laboratory measures the time for clot formation and then calculates the INR using the reagent's international sensitivity index and a mean normal prothrombin time, giving a standardized result.
The test requires a blood sample, and results are reported as the prothrombin time in seconds and the INR as a unitless number. When a broader clotting evaluation is needed, an activated partial thromboplastin time or fibrinogen level may be ordered at the same time. Reference ranges and target ranges depend on the reason for testing.
People may order this test when a healthcare provider is monitoring anticoagulant therapy, evaluating unexpected bleeding or bruising, or assessing liver-related clotting function. It is also used before some procedures. A qualified healthcare provider should interpret the result and manage any therapy adjustments; this test does not provide dosing guidance.
- Prothrombin time (PT) in seconds
- International normalized ratio (INR)
- Mean normal prothrombin time used in the calculation
- International sensitivity index of the testing reagent
- Activated partial thromboplastin time when included
- Fibrinogen level when included in an expanded panel
Why people order this test
- Monitor anticoagulant therapy when prescribed by a healthcare provider
- Evaluate unexpected bleeding or bruising
- Assess clotting function as part of a liver evaluation
- Check coagulation before certain procedures when requested
- Follow known clotting factor abnormalities over time
- Add detail to a broader coagulation workup
How to prepare
No fasting is usually required for this test. A blood sample is collected at the lab, and normal meals are generally fine. Tell your healthcare provider and the laboratory about medications you take, especially any that affect clotting.
Understanding your results
Results are typically available within one to three business days. The INR is interpreted against the range appropriate for the clinical situation, which is set by the ordering provider. Results should never be used to adjust therapy without professional guidance.
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Frequently Asked Questions
What is the INR?
The INR is a standardized way of reporting prothrombin time that adjusts for differences between laboratories and reagents, making results comparable across settings.
Why is prothrombin time tested?
It is used to evaluate how well the clotting system is working, to monitor certain anticoagulant therapies, and to assess clotting in conditions affecting the liver or clotting factors.
Does an abnormal INR mean I should change my medication?
No. Medication decisions must come from your healthcare provider. Never adjust anticoagulant therapy based on a laboratory result without professional guidance.
Do I need to fast before this test?
Fasting is usually not required. Follow any instructions provided with your order if other tests are included.
How is the sample collected?
A blood sample is drawn from a vein, usually in the arm, during a short lab visit.
Ready to get tested?
Order online and visit a collection site near you, or choose a mail-in kit. No referral and no insurance required.
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