Prothrombin Time (PT) test

 


🧪 1. Objective

The objective of this test was to measure the time it took for plasma to clot after adding tissue factor (thromboplastin) and calcium. It evaluated the functionality of the extrinsic and common pathways of the coagulation cascade.

 

🧪 2. Principle

The PT test was based on the principle that in the presence of thromboplastin and calcium, plasma forms a clot at a predictable rate. Delays in clot formation indicated deficiencies or inhibitors of factors I (fibrinogen), II (prothrombin), V, VII, and X. The result was reported in seconds and also converted into an INR (International Normalized Ratio).

 

🧪 3. Materials

  • Blood sample collected in a sodium citrate (blue top) tube

  • Calcium chloride solution

  • Thromboplastin reagent (tissue factor source)

  • Coagulometer or stopwatch

  • Incubator or water bath at 37°C

  • Gloves and lab timer

 

🧪 4. Procedure

  1. Blood was collected into a blue-top citrate tube and centrifuged to obtain platelet-poor plasma.

  2. The plasma was incubated at 37°C for 3 minutes.

  3. Thromboplastin and calcium were added to the plasma.

  4. The time to visible clot formation was measured in seconds using a coagulometer or stopwatch.

  5. The PT was compared with a control, and INR was calculated using the formula:
    INR = (Patient PT / Control PT)^ISI

 


🧪 5. Result (Example)

ParameterValueReference Range
PT (seconds)18.2 sec                            11–14 seconds
Control PT12.0 sec
INR1.9      0.8–1.2 (normal), 2–3 for anticoagulated patients




🧾 Interpretation:

Prolonged PT and elevated INR indicated a possible deficiency of clotting factors, liver dysfunction, warfarin therapy, or Vitamin K deficiency.

 

🧪 6. Uses

  • Monitored warfarin (Coumadin) anticoagulant therapy

  • Assessed liver function and clotting factor synthesis

  • Evaluated bleeding disorders

  • Screened patients before surgery for coagulation abnormalities

 

🧪 7. Conclusion
Prothrombin Time was a reliable and simple test to assess the extrinsic clotting pathway. It played a vital role in patient safety during anticoagulant therapy and in preoperative evaluations.

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