The prothrombin time (PT) allows the evaluation of the extrinsic pathway of the coagulation cascade and is widely used for the monitoring of patients under treatment with vita min K inhibitors. From the methodological point of view, it is quite simple: thromboplastin and calcium chloride are added to a plasma sample collected in sodium citrate (con centration 3.2%, corresponding to 0.105–0.109 mmol/L), and the clot formation time, expressed in seconds, is evaluated. The test can be performed either manually or using automated instruments. The PT is expressed as the ratio between the patient’s PT (expressed in seconds) and an average time obtained from a sample of subjects without coagulation alterations or under therapy with anticoagulant drugs, according to the following relation:

PT = PTpatient (seconds) / PTplasma pool

The test was first described by Quick in 1935. The central and defining element of the test is the use of thromboplastin, a mixture of tissue factor (animal extract or recombinant) and phospholipids. From the very first studies, it was evident that a standardization procedure was necessary in the preparation of thromboplastins since, depending on the preparation method, the results were extremely variable, making it difficult to evaluate patients over time. In 1977, the World Health Organization adopted the first thromboplastin calibration system, implementing an international reference preparation (IRP). This system was modified in 1982 with the introduction of the international sensitivity index (ISI) concept. Briefly, the calibration system was modified. In a group of subjects undergoing therapy with vitamin K antagonists, PTs performed simultaneously with both the reference preparation and the new commercially introduced thromboplastins were evaluated. The slope of the calibration line rep resents the ISI, which effectively establishes a relationship between the thromboplastin and the international reference preparation. Thus, the PT is expressed as an international normalized ratio (INR) according to the following relationship:

PT (INR) = [ PTpatient (s) PTplasma pool] ^ISI

Currently, INR is the universally accepted method for expressing PT independent of thromboplastin and is used by individual laboratories.

In the past, PT was performed manually, but soon automatic analyzers (commonly called coagulometers) became available. These, by means of an optical system or a mechanical system, allow for improved performance in terms of reproducibility and reliability of the analysis.

The ISI depends not only on the type of thromboplastin used but also on the analytical characteristics of the coagulometer, so thromboplastin manufacturers provide information regarding the ISI characteristics of thromboplastins related to the instrumentation used.

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مواضيع ذات صلة

Heparins and Fondaparinux

Activated Partial Thromboplastin Time (aPTT)

Diagnosis of Community-Acquired Pneumonia

progesterone receptor assay (PR assay, PRA, PgR)

progesterone assay

pregnancy-associated plasma protein-A (PAPP-A)

Laboratory Detection of Immunologic Responses

HPV DNA Detection

platelet volume, mean (Mean platelet volume [MPV])

platelet count (Thrombocyte count)

platelet antibody detection (Antiplatelet antibody detection)

Future Directions of Molecular Diagnostic Testing