The details of the method are beyond the scope of this text. The outline is as follows. Thromboplastin is added to plasma that has been separated from blood collected in sodium citrate and allowed time to mix. To this mixture calcium chloride is added. The time taken for the plasma to clot is the prothrombin time. The end point (clotting) may be determined manually or using automated (optical or magnetic) methods.
Source: All about blood
Image credits: All about blood
Art and Science of Laboratory Medicine