How to calculate prothrombin time activity

The prothrombin time activity (PTA) is calculated by the ratio of the prothrombin time (PT) to the normal control value, with the formula PTA=normal PT value/patient PT value x 100%. It is mainly used to evaluate exogenous coagulation function. The main influencing factors include warfarin use, vitamin K deficiency, liver disease, genetic coagulation factor deficiency, disseminated intravascular coagulation, etc.

1. Calculation formula: The core calculation formula for PTA is the normal PT value/patient PT value multiplied by 100%. The laboratory needs to first measure the average PT value of a healthy population as a normal control, which usually takes 11-13 seconds, and then input the patient's PT test results into the calculation. For example, when the patient's PT is 20 seconds and the normal control is 12 seconds, PTA=12/20 × 100%=60%. The lower the value, the worse the coagulation function.

2. Effects of Warfarin:

Oral anticoagulant warfarin can inhibit the synthesis of vitamin K-dependent coagulation factors II, VII, IX, and X, directly prolonging PT values. When PTA drops to 30% -40%, be alert to the risk of bleeding and adjust the drug dosage accordingly. Regular monitoring should be conducted during medication to maintain PTA within a safe range of 25% -45%.

3. Liver diseases:

Liver diseases such as cirrhosis can lead to reduced synthesis of coagulation factors, prolonged PT, and decreased PTA. If PTA remains below 50%, it may indicate decompensation of liver function, and a comprehensive evaluation should be conducted in combination with indicators such as albumin and bilirubin. In severe liver disease, PTA can be less than 20%, and fresh frozen plasma infusion is required to supplement coagulation factors.

4. Vitamin K deficiency:

Vitamin K is an essential cofactor for the synthesis of coagulation factors II, VII, IX, and X. Long term fasting, biliary obstruction, or broad-spectrum antibiotic use may lead to vitamin K deficiency, manifested as prolonged PT and decreased PTA. supplementing with 10mg intramuscular injection of vitamin K can improve PTA within 24 hours in mild deficiency.

5. Hereditary diseases:

Patients with hereditary coagulation factor VII deficiency have significantly prolonged PT, PTA can be lower than 10%, but activated partial thromboplastin time APTT is normal. This type of patient requires lifelong supplementation of recombinant coagulation factor VIIa or plasma products, and preoperative PTA should be increased to over 30%.

When monitoring PTA on a daily basis, it is necessary to maintain standardized blood collection to avoid hemolysis or coagulation affecting the results. Long term users of warfarin should maintain a stable intake of vitamin K and avoid consuming large amounts of foods rich in vitamin K, such as spinach and broccoli. Patients with abnormal liver function need to have regular check ups of coagulation function, and if necessary, supplement coagulation factor preparations under the guidance of a doctor. Before testing, it is necessary to inform the doctor of recent medication history, especially antibiotics, anticoagulants, and other drugs that may interfere with the test results.