A patient is placed on anticoagulant medication following total hip arthroplasty. Which laboratory value is most likely to be affected?

The correct choice relates to the prothrombin time, which is a crucial laboratory measurement that assesses the extrinsic pathway of coagulation and is specifically affected by anticoagulant medications such as warfarin. After a total hip arthroplasty, patients are often placed on anticoagulants to prevent venous thromboembolism (such as deep vein thrombosis). These medications work by inhibiting certain clotting factors that are dependent on vitamin K, effectively prolonging the prothrombin time.

As the patient starts anticoagulant therapy, the prothrombin time will be monitored and is expected to increase, reflecting the medication's effect on the coagulation process. Monitoring this value is crucial to ensure the patient remains within a therapeutic range and to help prevent complications associated with either clotting or excessive bleeding.

Hematocrit and hemoglobin levels may fluctuate due to surgery and blood loss, but they are not directly influenced by anticoagulant medications. The white blood cell count could also change due to surgical stress or infection but is not specifically monitored in relation to anticoagulation therapy. Hence, prothrombin time is the most relevant laboratory value that reflects the impact of anticoagulant treatment in this scenario.