PT and INR (Prothrombin)

What does PT/INR Test measure?

A PT measures the time it takes for a blood clot to form in a blood sample when specific substances (reagents) are introduced. Typically, the PT is conducted in conjunction with a partial thromboplastin time (PTT). These tests collectively evaluate both the quantity and functionality of coagulation factors, essential components in the formation of blood clots.

The PT is typically expressed in seconds and is compared to a reference range that represents normal PT values in healthy individuals. However, since the reagents used in PT testing can vary between labs and even within the same lab over time, these reference ranges can fluctuate. To standardize PT results across different labs globally, a committee from the World Health Organization (WHO) has developed and recommended the use of the Internationalized Normalized Ratio (INR). The INR is calculated based on the PT result and is particularly important for individuals taking the anticoagulant warfarin (Coumadin), prescribed for conditions such as deep vein thrombosis (DVT) and certain cardiovascular diseases (CVD) like atrial fibrillation. Warfarin "thins" the blood to prevent inappropriate clotting.

The INR is a calculated value that corrects for variations in PT reagents, enabling comparisons of results from different laboratories. Most labs provide both PT and INR values when performing a PT test. It's important to note that the INR is primarily relevant for those undergoing treatment with warfarin.

How is the test used?

The prothrombin time (PT), often used in conjunction with the partial thromboplastin time (PTT), plays a crucial role in diagnosing unexplained bleeding or inappropriate blood clotting. Additionally, the international normalized ratio (INR), derived from the PT results, is employed to monitor individuals receiving the blood-thinning medication warfarin (Coumadin).

These tests are instrumental in assessing the effectiveness of warfarin therapy, which is prescribed to prevent excessive clotting in various medical conditions. Healthcare practitioners prescribe warfarin and gauge its anticoagulant effects using PT/INR measurements. Dosing may be adjusted based on these results to maintain a balance that prevents clot formation without causing excessive bleeding. This requires careful monitoring.

Warfarin is commonly prescribed for conditions such as:

1. Irregular heartbeat (atrial fibrillation)
2. The presence of artificial heart valves
3. Deep vein thrombosis (DVT) and pulmonary embolism (PE)
4. Antiphospholipid syndrome
5. In certain cases of heart attacks with specific risk factors

The PT, often combined with the PTT, serves as an initial step in investigating bleeding or clotting disorders. Evaluating PT and PTT results together provides valuable insights into the potential disorder's nature. These tests, while not diagnostic on their own, typically guide healthcare providers on whether further testing is necessary.

Additional tests that may be conducted alongside PT and PTT, or in response to abnormal results, encompass:

1. Platelet count: Assessing platelet levels to detect potential reductions that can lead to excessive bleeding.
2. Fibrinogen testing: Conducted to rule out low fibrinogen levels or dysfunction as possible causes of prolonged PT.
3. Coagulation factor tests: These measure coagulation factor activity, helping identify reduced levels or factors with diminished functionality. In rare instances, antigen levels (quantity) of coagulation factors may also be measured.
4. von Willebrand factor testing: Occasionally ordered to investigate whether von Willebrand disease underlies prolonged PTT.
5. Lupus anticoagulant testing: May be recommended to further explore the reasons behind prolonged PTT and/or PT, especially in patients with clotting disorders.

Moreover, based on comprehensive patient histories, PT and PTT tests may be selectively performed as part of pre-surgical or pre-invasive procedure screenings to assess potential bleeding tendencies

When is this test typically requested?

A PT and INR are regularly requested when an individual is using the anticoagulant medication warfarin to ensure its effectiveness.

In cases where a person who is not taking anticoagulants experiences signs or symptoms of abnormal bleeding or clotting, a PT may be ordered. These signs and symptoms include:

1. Unexplained bleeding or easy bruising
2. Nosebleeds
3. Bleeding gums
4. The presence of a blood clot in a vein or artery
5. Acute conditions such as disseminated intravascular coagulation (DIC), which can lead to both bleeding and clotting due to rapid consumption of coagulation factors
6. Chronic conditions such as severe liver disease, which can impact hemostasis

Furthermore, PT, in conjunction with PTT, may be requested before surgery, especially when the procedure carries an elevated risk of blood loss. This is also considered when an individual has a medical history of bleeding issues, such as frequent or excessive nosebleeds and easy bruising, which could suggest an underlying bleeding disorder.

Which food and medications can influence PT and INR test outcomes?

Some antibiotics can increase PT and INR levels. Conversely, barbiturates, oral contraceptives, hormone-replacement therapy (HRT), and vitamin K (found in multivitamins or liquid nutrition supplements) may decrease PT.

Alcohol consumption can also impact PT results. Additionally, specific foods like beef and pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products contain significant amounts of vitamin K, which can influence PT results. It is crucial for your healthcare provider to be aware of all the medications, supplements, and foods you've recently consumed to ensure accurate interpretation and use of PT and INR results.

What would be the consequences of using warfarin with foods that are rich in vitamin K simultaneously?

Warfarin operates by limiting the available vitamin K necessary for the liver to produce various blood clotting factors. This means that warfarin and vitamin K oppose each other's actions. Drastic fluctuations in vitamin K intake can influence the effectiveness of warfarin in preventing blood clots without causing excessive bleeding. Instead of completely avoiding vitamin K-rich foods, it is crucial to maintain a steady and predictable daily intake of these foods. By ensuring a consistent amount of vitamin K consumption, you can receive the necessary vitamin K while effectively managing your warfarin therapy.

Is it necessary to have a PT done at the same time of day?

There is typically no specific requirement regarding the timing of your PT and INR measurements during the day. However, it is crucial to maintain a consistent daily schedule for taking your warfarin medication to ensure a steady level of the drug in your system. If your healthcare provider adjusts your warfarin dosage, they might recommend rechecking your blood within a few days to assess the impact of the dose change on your PT/INR levels. It's important to note that changes in dosage do not produce immediate effects on your PT/INR.