Prothrombin time (PT)

Prothrombin Time (PT)

INTENDED USE

Prothrombin Time (PT) is commonly used for screening for extrinsic factor deficiency, monitoring oral anticoagulant therapy and quantitative determination of the extrinsic coagulation factors. It is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot.

PRINCIPLE

Tissue thromboplastin, in the presence of calcium ions and Factor VII, activates the extrinsic pathway of coagulation. When a mixture of tissue thromboplastin and calcium ions is added to normal anticoagulant plasma, the clotting mechanism is initiated and a clot will form within a specified time period. If a deficiency exists within the extrinsic pathway, the time required for clot formation will be prolonged. The degree of prolongation is proportional to the severity of single factor deficiency, or in a cumulative deficiency of all the factors involved.

MATERIALS 


MATERIALS PROVIDED

        PT Reagent: ISI 1.0 


MATERIALS NEEDED BUT NOT PROVIDED: 

        Plasma Normal and Abnormal Control. 


STORAGE AND STABILITY

  • Do not mix or use the components of this kit with the components of any other kit with different lot numbers. 
  • Throughout testing all test tubes, syringes and pipettes should be plastic.
  • Throughout testing all test tubes, incubation time should be kept in constant and incubation temperature at 36.5-37.5°C.
  • If testing is delayed for more than 4 hours, plasma may be stored at 2-8°C.
  • Each laboratory should establish a Quality Control program that includes both normal and abnormal control plasmas to evaluate instrument, reagent tested daily prior to performing tests on patient plasmas. Monthly quality control charts are recommended to determine the mean and standard deviation of each of the daily control plasma. All assays should include controls, and if any of the controls are outside the established reference ranges, then the assay should be considered invalid and no patient results should be reported. SPECIMEN COLLECTION AND PREPARATION 1. Plasma obtained from whole blood samples that had been collected in a tube with 0.109M sodium citrate as an anticoagulant, nine parts of freshly collected whole blood should be immediately added to one part of anticoagulant. Centrifuge the whole blood specimen at 2500xg for 15 minutes. Separate the plasma using a plastic pipette and place it in a plastic test tube. Perform the Prothrombin Time assay within 4 hours. 2. Reconstitute the control plasmas (normal control plasma, abnormal control plasma) according to the package insert included with the control.


SPECIMEN COLLECTION AND PREPARATION

1. Plasma obtained from whole blood samples that had been collected in a tube with 0.109M sodium citrate as an anticoagulant, nine parts of freshly collected whole blood should be immediately added to one part of anticoagulant. Centrifuge the whole blood specimen at 2500xg for 15 minutes. Separate the plasma using a plastic pipette and place it in a plastic test tube. Perform the Prothrombin Time assay within 4 hours.
2. Reconstitute the control plasmas (normal control plasma, abnormal control plasma) according to the package insert included with the control.

PROCEDURE 


A. Manual Method 

1. Bring all reagents, controls and sample to room temperature 15 minutes prior to testing.
2. Pre-warm PT reagent at 37°C for 5 minutes.
3. Pipette 100µl of PT reagent to each tube.
4. Add 50 µl of sample, controls to the tubes prepared in step 3, start stop watch , mix in a water bath (37°C) for 8 seconds , then record the time required for clot formation . 

B.Automated Method 

To perform this test, refer to the appropriate Instrument Operator’s Manual for detailed instructions.

RESULTS


REFERENCE VALUES


Normal control sample: (11-16 seconds)
PTR: 1.0±0.15; INR: 0.8-1.24
  These values should only serve as guidelines.
 Because differences may exist between instruments,laboratories, and local populations, it is recommended that each laboratory establish its own reference range of expected Prothrombin Time LIQUID results.
 The results of the Prothrombin Time LIQUID tests should be reported to the nearest tenth of a second. Results greater than the upper limits of the range should be considered abnormal and follow-up testing should be performed. PT values below the lower limits of the range may indicate a compromised sample, and a new sample should be collected.

Table for INR values for PT Ratios between (0.05 - 6.0) 

ISI Value = 1.0


EXTRA NOTES:











How the Test is Performed


A blood sample is needed. If you are taking blood-thinning medicines, you will be watched for signs of bleeding.

How to Prepare for the Test

Certain medicines can change blood test results.

  • Your health care provider will tell you if you need to stop taking any medicines before you have this test. This may include aspirin, heparin, antihistamines, and vitamin C.
  • DO NOT stop or change your medicines without talking to your provider first.

Also, tell your provider if you're taking any herbal remedies.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

The prothrombin test is performed for the following purposes:

To monitor blood levels when the patient is taking blood-thinning medications like warfarin, which is used to treat deep vein thrombosis, pulmonary embolism

To detect the reason for abnormal blood clotting.



Furthermore, irregular clotting of blood happens due to numerous reasons, such as:


Liver diseases

Bleeding disorders like haemophilia

Vitamin K deficiency

Also, specific symptoms imply that a bleeding disorder may be present, such as:


Nosebleeds/epistaxis

Heavy blood flow during menstrual cycles in women

Bruising very often and easily

Discharge of blood along with urine

Incessant bleeding from wounds that does not stop despite employing pressure at the site of tissue injury.



The most common reason to perform this test is to monitor your levels when you are taking a blood-thinning medicine called warfarin. You are likely taking this medicine to prevent blood clots.

Your provider will check your PT regularly.

You may also need this test to:

  • Find the cause of abnormal bleeding or bruising
  • Check how well your liver is working
  • Look for signs of a blood clotting or bleeding disorder

Normal Results

PT is measured in seconds. Most of the time, results are given as what is called INR (international normalized ratio).

If you are not taking blood thinning medicines, such as warfarin, the normal range for your PT results is:

  • 11 to 13.5 seconds
  • INR of 0.8 to 1.1

If you are taking warfarin to prevent blood clots, your provider will most likely choose to keep your INR between 2.0 and 3.0.

Ask your provider what result is right for you.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

If you are not taking blood thinning medicines, such as warfarin, an INR result above 1.1 means your blood is clotting more slowly than normal. This may be due to:

  • Bleeding disorders, a group of conditions in which there is a problem with the body's blood clotting process.
  • Disorder in which the proteins that control blood clotting become over active (disseminated intravascular coagulation).
  • Liver disease.
  • Low level of vitamin K.

If you are taking warfarin to prevent clots, your provider will most likely choose to keep your INR between 2.0 and 3.0:

  • Depending on why you are taking the blood thinner, the desired level may be different.
  • Even when your INR stays between 2.0 and 3.0, you are more likely to have bleeding problems.
  • INR results higher than 3.0 may put you at even higher risk for bleeding.
  • INR results lower than 2.0 may put you at risk for developing a blood clot.

A PT result that is too high or too low in someone who is taking warfarin (Coumadin) may be due to:

  • The wrong dose of medicine
  • Drinking alcohol
  • Taking certain over-the-counter (OTC) medicines, vitamins, supplements, cold medicines, antibiotics, or other medicines
  • Eating food that changes the way the blood-thinning medicine works in your body

Your provider will teach you about taking warfarin (Coumadin) the proper way.

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight, but may include:

  • Fainting or feeling lightheaded
  • Hematoma (blood accumulating under the skin)
  • Multiple punctures to locate veins
  • Infection (a slight risk any time the skin is broken)

This test is often done on people who may have bleeding problems. Their risk of bleeding is slightly higher than for people without bleeding problems.

Alternative Names

PT; Pro-time; Anticoagulant-prothrombin time; Clotting time: protime; INR; International normalized ratio

Patient Instructions

References

Chernecky CC, Berger BJ. Prothrombin time (PT) and international normalized ratio (INR) - blood. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:930-935.

Lee GM, Ortel TL. Antithrombotic therapy. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 43.

Schafer AI. Approach to the patient with bleeding and thrombosis. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 162.

 

 

 

 

 

.https://drive.google.com/file/d/1G4hpp0C3s4uHYreSTfwC9922_K8nps88/view?usp=share_link

https://drive.google.com/file/d/1pEjIe2wwVTJWyxa-zr2eO1ojJMTWTCQc/view?usp=share_link

https://labpedia.net/prothombin-time-pt/#:~:text=Indications%20for%20Prothrombin%20Time%20(PT),-This%20is%20one&text=This%20test%20is%20done%20For,of%20bruises%20and%20abnormal%20bleeding.

https://www.medicine.mcgill.ca/physio/vlab/bloodlab/pt_ptt.htm

https://medlineplus.gov/ency/article/003652.htm 

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