What is the normal prothrombin time (PT) in seconds?

What is the normal prothrombin time (PT) in seconds?

• A. 8-12 seconds
• B. 10-13 seconds
• C. 15-20 seconds
• D. 20-25 seconds

Answer: (B)

Normal prothrombin time reflects how quickly the extrinsic pathway using vitamin K–dependent factors (II, VII, IX, X) clamps down to form a clot. In the lab, PT is measured by adding tissue factor and calcium to plasma and timing the clot’s formation. Because reagents differ between labs, reference ranges vary, but a commonly used normal window is about 10 to 13 seconds. This range indicates normal activity of the extrinsic pathway under standard testing conditions. If PT is longer than this, it can indicate anticoagulation with warfarin, liver disease, or factor deficiencies; PT is often expressed as INR to standardize results across labs.

Normal prothrombin time reflects how quickly the extrinsic pathway using vitamin K–dependent factors (II, VII, IX, X) clamps down to form a clot. In the lab, PT is measured by adding tissue factor and calcium to plasma and timing the clot’s formation. Because reagents differ between labs, reference ranges vary, but a commonly used normal window is about 10 to 13 seconds. This range indicates normal activity of the extrinsic pathway under standard testing conditions. If PT is longer than this, it can indicate anticoagulation with warfarin, liver disease, or factor deficiencies; PT is often expressed as INR to standardize results across labs.