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Background
Prothrombin time test is used to detect the common and extrinsic pathways of coagulation cascade. It measures the time of plasma to clot after the addition of calcium, phospholipid, and tissue factor. It also checks the functionality of clotting factor fibrinogen (I), prothrombin (II), V, VII, and X. Thromboplastin and calcium is an activator of extrinsic pathway. PT test is pivotal to diagnose the bleeding disease that monitor anticoagulant treatment and to assess the function of liver.
Prothrombin is a glycoprotein which is produced by liver that converts into thrombin during the clotting process. Thrombin converts the fibrinogen into fibrin, this can lead to the formation of the blood clot. PT assess how effectively these different clotting factors work to form a clot in reaction to an injury.
PT is more sensitive to deficiency of clotting factor VII than the deficiency of other factors in the final pathway. PT extensively differ from laboratory to laboratory depending on the instrument and reagents which are used. WHO created the INR (International normalized ratio) in 1983 to balance this variation because of the thromboplastin reagents and to increase the standardization of PT test in patients who are receiving the warfarin.
The INR is used to standardize PT result so that a PT result generated by one laboratory produces INR value that comparable to the other laboratories in the globe. It is a mathematical formula of the P result of patient which uses the sensitivity of reagents used in a laboratory by the ISI (International Sensitivity Index) given by the manufacturer. The ISI is a measurement of the sensitivity of the reagent to a reduced level of Vitamin K-dependent factors like IX, II, X, and VII. They are compared with the international reference preparation. The INR is calculated by the given formula:
INR = (Patient PT/Mean PT)
The 1st step to assess the prolonged PT/INR in patients who are not receiving warfarin, not have blood dyscrasia, and any medical condition which can lead to abnormal result need to repeat the test to exclude the laboratory error. Different factors affect the PT result and may lead to false prolonged PT result. A proper volume of blood is needed to get the accurate result. The amount of anticoagulant citrate in adult tube is 0.5 mL and the ratio of whole blood to citrate is about 9:1. The tube should be filled full or 60 to 80%.
The suggested plasma to citrate ratio is about 5:1. An increased hematocrit in diseases like myelodysplastic syndromes, polycythemia vera, or cyanotic congenital heart disease which can lead to decreased proportion in plasma of the sample compared to citrate may lead to false prolonged PT result. A proper amount of citrate and suggested amount of plasma to citrate is added to the blood sample in a glass tube.
A blood sample is collected after heparin bolus from the location above the infusion of heparin or by heparin-coated catheters can change the result of PT. Heparin acts as an anticoagulant by blocking the thrombin factor in final pathway. Heparin neutralizers are included into the thromboplastin reagents. It can neutralize to 2 U/mL of heparin. Excess heparin may disable the capacity of neutralizing agents and lead to prolonged PT.
Plasma turbidity may affect the PT result. The current method which is used to detect the clot formation is an automated photo-optical technique. It records the changes in the transmission of light. Excessive turbidity of plasma in diseases like severe hyperbilirubinemia, hyperlipidemia, and hemolysis may lead to abnormal result of PT.
INR is only used in patients who are receiving warfarin. Progressive liver disease is linked with different changes in the coagulation factors and the degree at which the prolonged PT or INR detects the bleeding risks.
Indications/Applications
PT test is used to diagnose and monitor different diseases which are linked with coagulation pathway.
To monitor the anticoagulation treatment: PT test is used to monitor the patient who is on the VKA (vitamin K antagonists) like warfarin.
To assess the function of the liver: The liver produces a clotting factor like prothrombin. PT is prolonged in patients who have liver diseases like hepatitis, cirrhosis because of the impaired synthesis of clotting factors.
To diagnose coagulation diseases like deficiency of vitamin K, disseminated intravascular coagulation (DIC), and deficiency of congenital factors.
Reference Range
The normal range of PT is about 11.0 to 12.5 seconds, 85 to 100%. The normal range is dependent on the regents.
Full anticoagulation treatment: Above 1.5 to 2 times than the control level, 20 to 30%
The normal level of INR is about 0.8 to 1.1.
The critical levels are:
PT test: 20 seconds
INR: Above 5.5
Interpretation
Different diseases may lead to short or prolonged PT test result.
Factors which can lead to prolonged PT result are:
Use of warfarin
Deficiency of vitamin K from biliary obstruction, malnutrition, use of antibiotics, or malabsorption syndrome
Liver disease like abnormal synthesis of clotting factor
Presence or deficiency of inhibitors to factors like fibrinogen, prothrombin/II, X, VII, V
Disseminated intravascular coagulopathy
Abnormality in fibrinogen like afibrinogenemia, hypofibrinogenemia, dysfibrinogenemia
After the administration of heparin bolus
Blood transfusion because of the dilution of plasma clotting protein
Hypothermia which is caused by the inhibition of cascade of enzymatic reactions of coagulation process
Factors which can lead to reduced PT result are:
Supplement of vitamin K
Transfusion of fresh frozen plasma
Collection And Panels
Sample type: Whole blood
Sample collection method: Routine venipuncture
Sample collection container: Blue top tube which contains 3.2 % concentration of citrate
Instructions: Fasting and special preparations are not needed before the test.
Storage instructions: The sample must be gently mixed by inversion techniques for 3 to 4 times after the collection. If the samples are kept at the room temperature on 22-24 ºC or at 2-4 ºC for 4 hours the procedure should be performed in 2 hours.
Panels: Other tests like partial thromboplastin time (PTT), complete blood count (CBC), and thrombin time (TT).
Modifying factor: Anticoagulants agents such as edoxaban, apixaban, rivaroxaban, dabigatran, and agratroban will change the result of PT test.
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