Prothrombin Time (PT/INR) Test
A prothrombin time test, often reported with a value called the INR, looks at how long a sample of blood takes to form a clot. Clinicians use it to help evaluate part of the blood's clotting process, and it is one of the more familiar tests connected to how blood clots.
About this test
Clotting is the body's way of sealing a wound, and it depends on a series of proteins in the blood called clotting factors working together in sequence. A prothrombin time test measures how long it takes for a clot to form under laboratory conditions, focusing on one of the pathways in that sequence.
The test is done with a routine blood draw at a patient service center. A phlebotomist collects a small sample from a vein, usually in the arm, and sends it to the laboratory.
Who might consider this test?
- Typically part of ongoing care rather than a one-time check
- The timing and frequency are directed by the prescribing clinician
Test details
| Quest code | 8847 |
|---|---|
| Methodology | Photo-Optical Clot Detection |
| Sample type | One full unopened 3.2% sodium citrate (light blue-top) tube |
| Collection | Quest patient service center |
Related tests and panels
Evaluates your body's ability to form blood clots safely and efficiently,…
Measures ABO blood group and Rh factor, information associated with Rh immune…
Measures red blood cells, white blood cells and platelets, a general panel used…
Quest resources
Additional details
What this test looks at
The test looks at the time it takes blood to clot through one of the body's clotting pathways, and it reports that as the prothrombin time, often alongside the INR. The INR is simply a way of expressing the result on a common scale so that different laboratories arrive at comparable values. Together they give a clinician a standardized look at part of the clotting process.
It also helps to remember what the test is not. It looks at one portion of a larger, interconnected clotting system rather than the whole of it. That is why a clinician may pair it with other clotting tests, and why the result is always read in the context of the bigger picture.
How it's often used in clinical care
One of the most common reasons this test is ordered is to monitor certain blood-thinning medications, which is done under a clinician's direction. In that context, the result helps the prescribing clinician keep an eye on how the medication is affecting clotting over time, and any adjustments are decisions for that clinician to make. This is not something to interpret or act on independently.
Outside of medication monitoring, a clinician may include a prothrombin time test as part of a broader look at clotting when exploring bleeding or clotting questions. In every case, it is read together with history, symptoms, and any other markers rather than on its own.
Order online and a physician order is included — no doctor's visit and no insurance needed. Your sample is collected at a Quest Diagnostics patient service center.
Laboratory results should always be interpreted by a licensed clinician who knows your health history. This page is information about a test, not medical advice.
Frequently asked questions
What does a prothrombin time (PT/INR) test look at?
It looks at how long a sample of blood takes to clot through one of the body's clotting pathways. The result is often reported as the INR, a standardized value that lets different laboratories produce comparable results.
What is the INR?
The international normalized ratio, or INR, is a way of expressing the prothrombin time on a common scale so results can be compared consistently across laboratories. A clinician interprets it in the context of your care.
Why is this test used to monitor medications?
Certain medications affect how the blood clots, and clinicians use the PT/INR to keep track of that over time. This monitoring is directed by the prescribing clinician, who decides on timing and any adjustments.
How is the sample collected?
Through a routine blood draw at a patient service center. A phlebotomist takes a small sample from a vein in the arm and sends it to the laboratory for analysis.
Should I change anything based on my result?
No. Results are interpreted by a clinician in context, and for anyone using this test to follow a medication, any decisions belong with the prescribing clinician rather than being acted on independently.
Can I get the Prothrombin Time with INR test without a doctor's visit or insurance?
Yes. The $75 cash price includes an independent physician's order, so no referral is needed, and insurance is never involved.