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Which laboratory value should a nurse monitor to determine the therapeutic effect of warfarin for a client with an artificial heart valve?

  1. Prothrombin time (PT)

  2. Partial thromboplastin time (PTT)

  3. INR

  4. Platelet count

The correct answer is: Prothrombin time (PT)

Prothrombin time is a laboratory value that measures how long it takes for the blood to clot. Warfarin is a medication that works to thin the blood and prevent clots, so monitoring the PT can determine if the warfarin is having its intended effect. Choice B, PTT, measures a different part of the clotting process and is not specifically affected by warfarin. Choice C, INR, is affected by warfarin and is commonly used to monitor its therapeutic effect, but PT is the more specific and direct indicator. Choice D, platelet count, measures the number of blood cells involved in clotting, but does not directly reflect the effectiveness of warfarin. Therefore, the correct answer is A, PT.