Thrombotic Disorders

Answer: D

Explanation: Because she had never had a problem achieving therapeutic INR in the past, a genetic polymorphism for warfarin resistance is unlikely. This should be considered in children in whom it is very difficult to anticoagulate initially with anything other than very high doses of warfarin. Alcohol use and the use of most antibiotics usually increase the INR. Missing one dose of warfarin does not typically decrease an INR between 2 and 3 down to 1.4. Discontinuing warfarin will usually drop the INR to < 1.5 in 3–5 days. Therefore, the most likely reason for this new warfarin resistance is the increase in vitamin K–containing foods in her diet.
Question 6

Answer: C

Explanation: UFH and its derivatives, including LMWH, exert their anticoagulant effects via potentiation of antithrombin activity, resulting in downregulation of thrombin and factor Xa. The terminal half-life of UFH is shorter than LMWH and this is a good feature if the baby had a high risk of bleeding or if he or she requires invasive procedures. Renal insufficiency reduces clearance of LMWH, requiring less frequent dosing or, in moderate/severe renal insufficiency, often the selection of an alternative anticoagulant. Any heparin derivative can induce HIT, including LMWH. Because this baby has renal insufficiency, the best choice is UFH until the renal disease corrects.
Question 7