Prothrombin Time (PT) is a laboratory test used to evaluate the extrinsic and common pathways of the coagulation cascade. The principle involves adding calcium-containing tissue thromboplastin to the patient's plasma, which activates the coagulation process. This leads to the conversion of prothrombin into thrombin, which in turn converts fibrinogen into fibrin, resulting in clot formation. The time it takes for the plasma to clot is measured. If any key factors in the extrinsic pathway are missing or reduced, 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:
1. **Preparation Before Testing:**
- **PT Reagent:** The reagent can be used directly after gently shaking it before use.
- **Sample Collection and Handling:** Fresh venous blood should be collected in a tube containing 3.2% sodium citrate at a ratio of 9:1. After mixing thoroughly, the sample is centrifuged at 3000 rpm for 15 minutes, preferably using a refrigerated centrifuge. The resulting plasma is then collected and tested within 2 hours to ensure accuracy.
2. **Testing Procedure:**
- **Manual or Semi-Automated Method:** Preheat the PT reagent to 37°C for at least 10 minutes but not longer than 30 minutes. Add 50 μL of the sample (or calibration or control plasmas) to a test tube and incubate at 37°C for 3 minutes. Then, mix with 100 μL of preheated reagent and start timing immediately. Record the time it takes for the plasma to clot.
- **Automated Coagulation Analyzer:** If using an automated instrument, follow the manufacturer’s instructions carefully.
3. **Result Interpretation and Influencing Factors:**
- **Result Analysis:** The Prothrombin Time Ratio (PTR) is calculated as the clotting time of the test plasma divided by the clotting time of the reference plasma. The International Normalized Ratio (INR) is derived from PTR using a specific sensitivity index (ISI): INR = PTR^ISI.
- **Influencing Factors:** Several variables can affect the PT result, including the method of testing, temperature, pH, sample collection technique, type of anticoagulant used, time between collection and testing, storage conditions, presence of oral anticoagulants (such as warfarin), or other medications that may interfere with coagulation.
It is essential to maintain consistent procedures and consider all potential variables to ensure accurate and reliable results. Proper training and attention to detail are crucial when performing and interpreting PT tests.
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