A hospitalized patient receiving heparin therapy for DVT (Deep Vein Thrombosis) prophylaxis develops platelet count of 68,000/µl and reports new unilateral calf pain. What is the nurse's priority action?

The clinical picture described — thrombocytopenia with a platelet count of 68,000/µl and new unilateral calf pain in a patient receiving heparin — is highly consistent with Heparin-Induced Thrombocytopenia type II (HIT). HIT is an immune-mediated, life-threatening adverse reaction where antibodies form against platelet factor 4–heparin complexes, activating platelets and causing paradoxical thrombosis despite low platelet counts. The hallmark is a drop in platelet count of more than 50% from baseline occurring 5–10 days after heparin initiation, combined with new thrombotic events. The priority action is to immediately discontinue all heparin — including heparin flushes and heparin-coated catheters — and notify the provider so that a non-heparin anticoagulant (such as argatroban or bivalirudin) can be started to treat the underlying thrombosis. Continuing heparin and monitoring is dangerous because continued heparin exposure perpetuates antibody activation, increasing the risk of catastrophic thrombosis. Administering vitamin K is the reversal agent for warfarin, not heparin, and is entirely inappropriate here. Applying a warm compress to the affected limb is contraindicated in suspected DVT or HIT because manipulation can dislodge a thrombus and does not address the underlying immune-mediated process. Stopping heparin immediately is the single most critical nursing action in suspected HIT.