Prothrombin time (PT) evaluates the activity of the factors involved in the “extrinsic” and “common” clotting path: FVII-proconvertine, Stuart-prower FX-factor, FV-proaccelerine, BE-BE-prothrombin and fibrinogen. Coagulation factors II, VII, IX and X are synthesized in the liver as inactive (Pivka). These proteins (proteins with C, S and Z) have in common unique residues glutamic acid-γ-carboxylic acid (Gla) from the N-terminus of the molecule, which require the presence of vitamin K for the synthesis and which are essential for calcium binding, serving the deck surface protein binding to phospholipids or to be functional.

Thus PT evaluates both activity-dependent coagulation factors Vitamin K (less FIX), factor V and fibrinogen, protein synthesis and function of the liver, diagnostic and therapeutic implications.

In vivo, the main way to initiate blood clotting is extrinsic system, which includes components of blood and vascular elements, coagulation occurring when tissue factor (TF) is bound to FVIIa. FVIIa-TF complex enzyme activates both FIX and FX, FX being  a better substrate. FXa interacts with the cofactor or FVA to form the prothrombinase complex, just  enough to generate a very small amount of thrombin in the vicinity of TF expressing cells.

In vitro, PT measures the elapsed time between adding a standardized mixtures of tissue thromboplastin and calcium to a plasma aticoagulated with, platelet-poor and detection of clot formation, representing polymerization of fibrin, resulting from the action of thrombin.

Tissue thromboplastin is a mixture of phospholipid vesicles and tissue factor. Tissue factor is absent in normal plasma, it must be supplied from an external source and thus triggered cascade of enzymatic reactions PT is known as “extrinsic pathway”. Traditionally thromboplastin tissue was obtained from animal tissue extract (brain, placenta, lung), but today there are available mixtures of recombinant human tissue factor and purified phospholipids, which allow preparation of well defined thromboplastin, having the advantage of the lack of contamination factors coagulation (which can be found in tissue factor extracted from other sources), thus serving the increase of PT sensitivity to deficiencies of coagulation factors.

The minimum amount of thrombin generated by activation of the extrinsic pathway by tissue factor is sufficient to quickly produce  a detectable clot- by PT test. Enhanced thrombin generation associated with the second phase of coagulation is not necessary to produce a normal PT. This explains that the intrinsic pathway deficiencies of factors VIII, IX and XI (and of FXIIa and FXIIa) don’t produce the extension of PT. Physiological coagulation requires the second phase, explosive thrombin generation, but not clot formation in PT.

A prolonged PT indicating deficiency of one or more clotting factors (I, II, V, VII, X) or presence of inhibitor.

Recommendations for testing:

-PT Is the most common test used to monitor oral anticoagulant therapy (warfarin), which is sensitive to three of the four factors of vitamin K-dependent coagulation;

– Screening for congenital or acquired deficiencies of factors II, V, VII, X and fibrinogen;

– Disfibrinogenemias;

– The presence of inhibitors to coagulation factors;

– Vitamin K;

– Monitoring of liver function protein synthesis;

– Preoperative screening of hemostasis.

Expression of results

– The clotting time – in seconds;

– As a percentage (%) of normal activity = prothrombin activity (PA); the measurable = 10-100%;

– Prothrombin ratio (PR = PT patient sec / sec normal plasma PT);

– The INR (International Normalized Ratio).

Numerous international studies have shown that in the stable phase of therapy with oral anticoagulants (ACO) results can vary significantly depending on the tissue of origin of the thromboplastin and instrument used for their determination. To solve this problem, the World Health Organization 1982 introduced a standardized international validation procedure thromboplastines. The results obtained by this procedure during anticoagulant stabilized therapy, do not depend on the reagent used. Thus the prothrombin report  is converted to INR (International Normalized Ratio) according to the formula:

INR = (PTpacient / PTplasma normal) ^ ISI

ISI = international sensitivity index (International Sensitivity Index) thromboplastin used, calculated in relation to the reference thromboplastin ISI = 1. ISI values ​​used in the world of tromboplastines  vary between 1 and 3 and are set by manufacturers for each batch of reagents. As ISI value is closer to 1, the more reagent sensitivity to therapy with ACO is higher.

INR is only useful for in patients with stable oral anticoagulant therapy and has no value for the diagnosis or treatment of those with prolonged PT because of other reasons.

INR is less accurate when used from the start of treatment with ACO. However it is more reliable than unconverted PT ratio and is recommended to be used both in during treatment initiation and maintenance of ACO.

The reagent used in the laboratory Synevo contains a thromboplastin derived from rabbit brain, with ISI = 1.2-1.4.

Reference values

             – Therapeutic range:

INR = 2.0-3.0 (2.5) for most clinical situations: prevention and treatment of venous thrombosis; treatment of pulmonary embolism; prevention of systemic embolism in patients with atrial fibrillation, myocardial infarction (if some aspirin), rheumatic mitral valve disease, mitral valve prolapse, mitral annular calcficare, mobile aortic arch thrombus, biological heart valves or prosthetic aortic mechanical bivalvular; hip and knee surgery; venous sinus thrombosis; antiphospholipid syndrome.

INR = 2.5-3.5 (3.0) mechanical heart valves, repeated thromboembolic episodes in patients with INR therapeutic anticoagulation.

INR = 3.0-4.0 (3.5): myocardial infarction, thrombosis patients with mechanical aortic valve prosthesis remission.

INR = 3.5-4.5 (4.0): patients with thrombosis of mechanical mitral valve prosthesis remission.

INR = 1.5-1.9: patients with a first unprovoked episode of deep vein thrombosis or pulmonary embolism after the first 3 months of treatment, where it is not possible INR testing every 4 weeks for monitoring of treatment; primary prevention of myocardial infarction in high-risk patients.

Anticoagulant treatment- laboratory control

The anticoagulant effect stabilizes after 4-5 days of treatment initiation. According to ACCP recommendations, monitoring PT / INR should start after the first 2-3 doses of oral anticoagulant and PT should be measured daily until the results of PT / INR stabilizes in the therapeutic range. When necessary a rapid anticoagulant effect heparin or low molecular weight heparin should be administered concomitantly, and the overlap with ACO lasts until the INR was in the therapeutic range for 2 consecutive days.

Subsequent determinations frequency is determined by the physician according to the patient’s compliance and response to treatment. After obtaining a stable dose monitoring is recommended at intervals not exceeding 4 weeks. Frequency should be increased whenever determinations are introduced or discontinue medications that could influence the effectiveness of anticoagulation.

It is recommended that a single laboratory method will be used for monitoring the treatment of each patient.

Critically low values: INR> 6 – bleeding risk (especially in patients with gastrointestinal diseases, hypertension, renal disease, cerebrovascular disease, antiplatelet therapy, potential, other drugs).

Interpretation of results

Prolonged PT (AP low): treatment with oral anticoagulants, congenital or acquired deficiency of factors II, V, VII, X, vitamin K deficiency in infants, newborn bleeding disorders, liver, biliary obstruction, intestinal absorption disorders fat (eg .: sprue, celiac disease, chronic diarrhea), malnutrition, CID, Zollinger-Ellison syndrome, hypofibrinogenemia (fibrinogen <60 mg / dL), disfibrinogenemia, the presence of circulating anticoagulants (specific inhibitors to factors extrinsic pathway are rare), some patients with Ac lupus anticoagulant that link and increase FII, clearance amyloidosis (with low levels of FX).