What is the significance of prothrombin time?

Under normal circumstances, prothrombin time (PT) is the key indicator reflecting the function of the extrinsic coagulation pathway. The normal reference range is typically 11–13 seconds. Its clinical significance includes evaluating extrinsic coagulation pathway function, monitoring the efficacy of oral anticoagulant therapy, assessing the severity of liver disease, screening for vitamin K deficiency, and aiding in the diagnosis of disseminated intravascular coagulation (DIC). Detailed analysis is as follows:

1. Evaluation of extrinsic coagulation pathway function: Prothrombin time primarily assesses the activity of coagulation factors II, V, VII, X, and fibrinogen. When these factors are deficient—such as in vitamin K deficiency, reduced or impaired liver synthetic function—PT becomes prolonged, indicating obstruction in the extrinsic coagulation pathway and an increased risk of bleeding.

2. Monitoring efficacy of oral anticoagulant therapy: When taking oral anticoagulants such as warfarin, dosage adjustments are guided by PT results, usually expressed as the International Normalized Ratio (INR). Maintaining INR between 2.0 and 3.0 effectively prevents thrombosis while minimizing excessive bleeding risk. Any significant prolongation or shortening of PT requires timely dose adjustment to ensure safe medication use.

3. Assessing severity of liver disease: Coagulation factors II, VII, IX, and X are all synthesized in the liver. In conditions involving extensive hepatocyte damage—such as cirrhosis or severe hepatitis—synthesis of these factors decreases, leading to prolonged PT. The degree of PT prolongation correlates positively with the severity of liver injury, making it an important parameter for evaluating liver function classification and assisting in prognosis assessment.

4. Screening for vitamin K deficiency: Vitamin K is an essential cofactor for the synthesis of coagulation factors II, VII, IX, and X. Deficiency may result from inadequate dietary intake, malabsorption, or prolonged antibiotic use, leading to prolonged PT. Measuring PT can help detect vitamin K deficiency early, allowing prompt supplementation to correct coagulation abnormalities.

5. Aiding in the diagnosis of disseminated intravascular coagulation (DIC): During the consumptive hypocoagulable phase of DIC, massive consumption of coagulation factors leads to significantly prolonged PT, often accompanied by other abnormalities such as thrombocytopenia and decreased fibrinogen levels.

It should be noted, however, that while PT is a fundamental test for evaluating coagulation function, its results must be interpreted in conjunction with clinical symptoms and other coagulation parameters, and should not be assessed in isolation.