How often should I check my INR on warfarin?

When starting warfarin, INR is typically checked every few days until stable. Once stable, most patients test every 2-4 weeks. Some patients on long-term therapy may test monthly. Your doctor will determine the right frequency for you.

What can affect my INR results?

Many things affect INR: vitamin K-rich foods (leafy greens), alcohol, many medications (antibiotics, NSAIDs), supplements (fish oil, vitamin E), illness, and changes in diet. Consistency is key - try to keep vitamin K intake steady rather than avoiding it completely.

What is the difference between PT and INR?

PT is the actual time in seconds for your blood to clot. INR converts this to a standardized ratio so results can be compared across different labs and testing methods. INR is the preferred measure for monitoring warfarin therapy.

Can I get this test if I am not on blood thinners?

Yes. PT/INR is also useful for investigating unexplained bleeding, assessing liver function, or evaluating clotting before surgery. It is not exclusively for warfarin patients.

Back to Tests

✨ General Health

Prothrombin Time (PT/INR) - Blood Clotting Test

PT/INR

Measures how quickly your blood clots. Essential for anyone on blood thinners like warfarin, or investigating unexplained bleeding or bruising.

The Prothrombin Time (PT) test measures how long it takes for your blood to clot. The result is also expressed as an INR (International Normalized Ratio), which standardizes results across different laboratories.

This test evaluates the extrinsic and common pathways of blood coagulation, specifically measuring the function of clotting factors I (fibrinogen), II (prothrombin), V, VII, and X.

For people taking warfarin (Coumadin), regular PT/INR monitoring is critical to ensure the medication is working properly without increasing bleeding risk.

Price

$10

All-in

$28

incl. $18 lab & physician fee

Results

1 business days

Fasting

Not Required

What's Measured

INR

What This Test Measures

This test measures two related values:

PT (Prothrombin Time): The time in seconds for blood to clot (normal: 11-13.5 seconds)
INR (International Normalized Ratio): A standardized ratio comparing your clotting time to normal (target varies by condition)

For patients NOT on blood thinners: Normal INR is 0.8-1.2 For patients ON warfarin: Target INR is typically 2.0-3.0 (or 2.5-3.5 for mechanical heart valves)

Who Should Consider This Test

Patients taking warfarin (Coumadin) or other vitamin K antagonists who need regular monitoring. Also useful for people with unexplained bleeding, easy bruising, frequent nosebleeds, or heavy menstrual periods. Commonly ordered before surgery to assess bleeding risk, or to evaluate liver function since the liver produces clotting factors.

Health Insights

Conditions Screened

Bleeding disordersVitamin K deficiencyLiver diseaseWarfarin effect monitoringDIC (Disseminated Intravascular Coagulation)Factor deficiencies

Related Symptoms

Easy bruisingProlonged bleeding from cutsFrequent nosebleedsBlood in urine or stoolHeavy menstrual periodsUnexplained bleeding

When to Get Tested

No fasting required. If on warfarin, test at the same time of day relative to your dose for consistent results. Your doctor will advise on testing frequency - typically weekly when starting warfarin, then monthly once stable.

No fasting needed. How to prepare for your blood test →

Understanding Your Results

For patients NOT on blood thinners:

INR 0.8-1.2: Normal
INR above 1.2: May indicate liver disease, vitamin K deficiency, or clotting factor issues

For patients ON warfarin:

INR below 2.0: Blood may clot too easily (undertreated)
INR 2.0-3.0: Therapeutic range for most conditions
INR above 3.0: Increased bleeding risk (may need dose adjustment)
INR above 4.0: Significantly elevated bleeding risk - contact your doctor

What to Expect

After placing your order, we send it straight to Quest Diagnostics — there is no consent form to sign and no partner email to wait for. Your requisition is usually ready to download from your dashboard within about an hour, and you can book your blood draw at any Quest Diagnostics location.

The blood draw typically takes just a few minutes. Results are usually available within 1 business days and will appear in your secure JustLabs dashboard.

Our results include reference ranges and easy-to-understand indicators showing whether your values are within normal limits.

How much does the PT/INR test cost?

The PT/INR test costs $10 at JustLabs — $28 all-in, since the $18 lab & physician order fee is charged once per order, however many tests you add. The price is fixed before your blood draw; no insurance or doctor visit needed.