PT/INR Blood Test: How It Works and Do I Need to Do It

What is a PT/INR blood test?

A prothrombin time (PT) is a blood test that measures how long (in seconds) it takes your blood to clot. Prothrombin itself is one of several proteins, called clotting or coagulation factors, produced by the liver that can trigger blood clotting to stop bleeding after an injury.

However, PT findings may vary between laboratories depending on the method used. To account for this variability and provide consistent results for individuals taking blood-thinning medications, such as warfarin, the international normalised ratio (INR) is used as the standard reporting format for prothrombin time test results.

This blood test can be done as a stand-alone test or in combination with another blood test called the partial thromboplastin time (PTT) test, which assesses how other clotting factors in your blood work to form blood clots. 

In Singapore, the PT/INR test is widely available in hospitals or clinics and can also be done at home using a test kit.

What is the PT/INR test used for?

Your healthcare provider uses a PT/INR blood test to determine whether your blood is clotting properly. If your blood clotting is too slow, you may be at risk of excessive bleeding. On the other hand, if it is too fast, it can lead to blockages in the blood vessels.

This assessment is particularly important when adjusting the dose of anticoagulant (blood-thinning) medication and for assessing other conditions that affect your blood's ability to clot. Here are some conditions that require regular PT/INR monitoring, including:

Monitoring warfarin medications

• If you are currently taking warfarin, your healthcare provider will regularly use a PT/INR test to monitor your blood clotting time and adjust your dose accordingly. This procedure helps prevent dangerous clots from forming in your blood vessels while ensuring that your blood can still clot properly when needed.

Pre-surgical evaluation

• Before surgery or other invasive procedures, your healthcare provider may use a PT/INR test to ensure your blood clots properly so you are not at risk of excessive bleeding during or after surgery.

Liver function assessment

• Since the liver is responsible for producing clotting factors, an abnormal PT/INR result could signal liver conditions like cirrhosis.

• Your healthcare provider often includes this blood test in a liver health screening to evaluate liver health and monitor disease progression.

Evaluating vitamin K levels

• Vitamin K is essential for the production of blood clotting factors, and a PT/INR test can show if a deficiency of this vitamin is causing prolonged clotting times. 

• It can also check whether taking vitamin K supplements or eating a diet rich in vitamin K is causing your blood to clot more quickly.

When do I need this test?

Your healthcare provider may recommend this blood test if you have symptoms of a bleeding or clotting disorder. Additionally, doctors also use it to monitor medical conditions that affect your body's ability to properly regulate blood clotting.

Here are some symptoms and conditions that might require this test:

Symptoms of a bleeding disorder

• Frequent bruising

• Unexplained bleeding, such as nosebleeds or gum bleeding

• Prolonged bleeding from cuts

• Swollen or painful joints

• Blood in stool or urine

• Heavy menstrual periods in women

Symptoms of a clotting disorder

• Leg pain

• Redness or darkening of the skin on the legs

• Warm sensation on the skin

• Shortness of breath and chest pain

• Rapid heartbeat

Monitoring specific health conditions

• Atrial fibrillation, an irregular and rapid heartbeat, can lead to the formation of blood clots and increase the risk of stroke.

• Deep vein thrombosis (DVT), a condition where blood clots form in deep veins, usually in the legs, which can be dangerous if they travel to the lungs.

• Pulmonary embolism, where a blood clot that forms in a blood vessel elsewhere in the body (often in the leg) travels to an artery in the lung and forms a blockage

• Haemophilia is a genetic disorder where the blood lacks some clotting factors, leading to prolonged bleeding even from minor injuries.

• Von Willebrand  disease is a genetic disorder where individuals have low levels of von Willebrand factor, which is one of the blood coagulation factors.

If you are experiencing any of the mentioned symptoms, request an appointment with Thomson Medical. Our specialists can assist with further diagnosis, including a PT/INR blood test, to determine the underlying cause of your symptoms and provide a tailored appropriate treatment plan.

What to expect during a PT/INR blood test?

The PT/INR is a simple test that only requires your blood sample, which can be taken at the clinic or at home.

At the clinic

If your blood is taken at the clinic, your healthcare professional will use a small needle to draw a  blood sample from a vein in your arm. Once the needle is inserted, a small amount of blood is collected into a test tube or vial. 

Once the sample has been taken, the needle will be removed, and a piece of gauze and a bandage will be placed over the puncture site. Your blood sample will then be checked to see how long it takes for your blood to clot. You can usually expect the results on the same day.

At home

If you routinely take blood-thinning medications such as warfarin, you will need regular monitoring. To help you save time from frequent visits to the clinic, your healthcare professional may suggest using a PT/INR test kit at home to monitor your treatment remotely.

The home test involves taking a drop of blood from your fingertip and placing it on a test strip. The PT/INR test kit then provides results within minutes. After that, you’ll review these results with your healthcare professional via telemedicine to ensure accurate monitoring.

How to prepare for the test?

To ensure accurate test results, there are some preparations you need to make, including:

• Maintain a consistent diet

  • Since vitamin K helps blood clot, sudden changes in its consumption may affect your test results. 

  • So it's important to maintain a consistent daily intake of vitamin K-rich foods such as leafy greens (like spinach, kale, and broccoli) and Brussels sprouts, or vitamin K supplements.

• Avoid certain medications

  • Before the tests, certain medications should be avoided, as they can influence your PT/INR results. For instance, some antibiotics can raise PT/INR levels, while oral contraceptives and hormone replacement therapies might lower your PT results.

• Stay hydrated and avoid strenuous exercise

  • Dehydration or vigorous exercise can affect blood consistency, so you need to drink enough water and get enough rest before your test.

• Schedule testing at the same time

  • As PT/INR levels can fluctuate throughout the day, if you test regularly at home, try to test at the same time of day for consistent results.

Inform your doctor about all medications, vitamins, herbal remedies, or supplements you are taking, including over-the-counter options, to ensure accurate assessment. If you're taking a blood thinner, continue to take your daily dose unless recommended otherwise.

What do your INR results mean?

The INR result helps your healthcare provider to assess how well your blood is clotting. What is considered a normal INR will depend on whether you're taking blood-thinning medication or not:

• The normal blood clotting range for someone not taking blood thinners is 0.8–1.1 or a prothrombin time of 10-14 seconds.

• If you are taking warfarin or other anticoagulants, the normal INR range is 2.0–3.0. However, this range may vary depending on your medical conditions.

If your test result is lower or higher than the above reference range, it may indicate an abnormal blood clot.

Low INR

If your INR is below the reference range, your blood clots too quickly, increasing the risk of forming  blood clots and conditions such as stroke, heart attack, deep vein thrombosis, and pulmonary embolism.

This condition can be caused by several conditions, such as:

• The clotting disorder that causes your body to form excessive clots in the arteries or veins.

• High intake of vitamin K, through supplements or foods such as spinach, kale, turnip greens and broccoli.

• Interactions between medications or supplements that decrease the effectiveness of blood thinners.

• Missed or incorrect doses of blood thinners.

• Oestrogen-containing medicines, such as birth control pills or hormone replacement therapy, that could increase clotting.

High INR

If your test results are higher than the normal range, this means that your blood may clot too slowly. This increases the risk of excessive bleeding because your blood takes longer than normal to form a clot.

This condition can be caused by several things, such as:

• A bleeding disorder occurs when your body is unable to clot blood effectively.

• Vitamin K deficiency, which causes the blood to be unable to clot properly.

• Taking too high a dose of warfarin, making the blood harder to clot.

• Excessive dosage of blood thinners.

• Liver diseases that reduce the clotting factor production.

• Interactions between medications or supplements that increase the effectiveness of blood thinners.

If your PT/INR results are outside the normal range, your doctor may adjust your anticoagulant dosage or recommend dietary changes. By understanding your PT/INR results, you can better manage your health and reduce risks associated with abnormal clotting.

Whether you're on anticoagulant medications or not, if your PT/INR are lower or higher than the reference range, don't hesitate to consult a doctor. Request an appointment with Thomson Medical for further evaluation and recommendations based on your medical conditions.

PT/INR costs in Singapore

In Singapore, a PT/INR blood test can be performed at a hospital, polyclinic, or private GP clinic. The cost of this imaging test depends on whether it's performed as a standalone test or as part of a health screening package, as well as the choice of medical facility.

To help cover the cost, you can use up to SGD 500 annually from your MediSave account for non-complex chronic conditions or SGD 700 annually for complex chronic conditions to cover outpatient treatments, including the PT/INR test. If you have private insurance, check with your provider to see if this blood test is covered under your policy.

For detailed fee information and payment options, please consult your healthcare provider directly. Request an appointment with our specialists at Thomson Medical today for a detailed price breakdown and a personalised care plan.

Are there any risks to the test?

The PT/INR test is a very low-risk procedure, as it only requires a sample of your blood. However, you may experience some minor discomfort, such as when blood is taken directly from a vein or when the skin of your finger is punctured.

• Mild pain or discomfort around the punctured area.

• A slight sting during the needle insertion or removal.

• Dizziness due to the withdrawal of blood.

• Slight bruising or bleeding at the needle insertion site, especially if you're on blood thinners.

FAQ

What is the PT/INR blood test?

Prothrombin time (PT) is a blood test that measures how long it takes your blood to clot, while the INR (International Normalised Ratio) standardises the results of the PT (Prothrombin Time) test.

INR allows for consistent reading of test results across different laboratories, which is important for monitoring and dosing in patients taking anticoagulant drugs such as warfarin.

What is a safe level of INR?

A safe INR level depends on whether you are taking anticoagulants and your medical condition. For people who are not taking anticoagulants, the normal INR range is 0.8–1.1. Meanwhile, for people taking warfarin or other anticoagulants, the target INR range is usually 2.0–3.0.

However, your healthcare provider may recommend a different target range based on your specific medical condition. Some conditions may require a slightly higher or lower target INR for the best treatment results.

What happens if your INR is too high?

If your INR is higher than your reference range, it means your blood is taking longer than normal to clot, which increases your risk of bleeding. This condition can cause symptoms such as:

• Frequent bruising

• Prolonged bleeding from cuts or wounds

• Nosebleeds or bleeding gums

• Blood in stool or urine

• Internal bleeding in severe cases

If you experience unusual bleeding or bruising, contact your healthcare provider. They will then assess your condition and may adjust your warfarin dose or recommend other medical treatments.

What should I eat if my INR is high?

If your INR level is high, your healthcare provider may recommend increasing your intake of vitamin K, as this vitamin helps blood clotting and can lower your INR. Foods that are rich in vitamin K include:

• Leafy green vegetables (spinach, kale, lettuce)

• Brussels sprouts

• Broccoli

• Cabbage

• Green tea

However, it's important to maintain a consistent intake of these foods rather than making sudden changes to your vitamin K intake. This is because sudden increases or decreases in vitamin K consumption can lead to fluctuations in your INR levels.

Always talk to your doctor before making any major changes to your diet, especially if you're taking anticoagulant medication.

Does lack of sleep affect INR?

While sleep deprivation itself may not directly change your INR, it can influence factors that indirectly affect your blood clotting and medication metabolism, such as:

• Stress and hormonal changes

  • Poor sleep can increase stress levels, which may affect your body's metabolism and how it processes medications like warfarin.

• Dietary changes

  • Lack of sleep might lead to poor eating habits, such as skipping meals or consuming more caffeine or alcohol, which can affect INR levels.

• Medication adherence

  • Lack of sleep can lead to fatigue, which causes you to skip warfarin doses or take them at irregular times, leading to unstable INR levels.

• Liver function

  • Warfarin is metabolised in the liver, and sleep deprivation can temporarily affect liver function, possibly altering how your body processes the medication.

• Inflammation and illness

  • Chronic lack of sleep can weaken your immune system, making you more susceptible to infections or inflammation, which can affect INR.

The information provided is intended for general guidance only and should not be considered medical advice. For personalised recommendations based on your medical conditions, request an appointment with Thomson Medical.