Prothrombin time (PT) is a laboratory test used to evaluate the extrinsic and common coagulation pathways. The principle involves adding calcium-containing tissue thromboplastin to the plasma, which initiates the coagulation cascade. This leads to the conversion of prothrombin into thrombin, which then transforms fibrinogen into fibrin, resulting in clot formation within a specific timeframe. If any essential factors in the extrinsic pathway are deficient, the clotting time will be prolonged. The extent of this prolongation correlates with the severity of the factor deficiency or overall impairment in the coagulation system.
The testing procedure includes several key steps. First, pre-testing preparation is crucial. The PT reagent should be gently shaken before use and can be applied directly. For sample collection, fresh venous blood is mixed with trisodium citrate in a 9:1 ratio to prevent clotting. After mixing, the sample is centrifuged at 3000 rpm for 15 minutes, ideally using a cryogenic centrifuge. The resulting plasma is collected and tested within two hours to ensure accuracy.
During the detection process, there are both manual and automated methods. In manual procedures, the PT reagent is pre-warmed to 37°C for at least 10 minutes but not exceeding 30 minutes. A 50 μL sample (or calibration or control plasma) is incubated at 37°C for three minutes, followed by the addition of 100 μL of pre-warmed reagent. The time taken for clot formation is recorded immediately. For automated systems, the operation should follow the manufacturer's guidelines provided in the instrument manual.
Interpretation of results involves calculating the Prothrombin Time Ratio (PTR), which is the ratio of the patient’s clotting time to that of a normal reference plasma. The International Normalized Ratio (INR) is then calculated using the formula INR = PTR^S, where S is the international sensitivity index of the reagent used.
Several factors can influence PT results, including the testing method, ambient temperature, pH levels, sampling technique, type of anticoagulant used, sample handling, storage conditions, and the presence of oral anticoagulants or other medications. It is important to maintain standardization and consistency throughout the entire testing process to ensure reliable and reproducible outcomes.
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