Low-Molecular-Weight Heparin / Enoxaparin

A patient's platelet count drops 50% five days into a heparin drip — this isn't just thrombocytopenia, it's a paradoxical clotting emergency that kills if you keep infusing.

Unfractionated heparin (UFH) is monitored with the activated partial thromboplastin time (aPTT), with a therapeutic target typically 1.5–2.5 times the control value (roughly 46–70 seconds, institution-dependent). aPTT is drawn 6 hours after initiation or any dose change because that's when steady state is reached. Low-molecular-weight heparin (LMWH) like enoxaparin generally does not require routine aPTT monitoring — if checked at all, anti-Xa levels are used. The most dangerous adverse effect of any heparin is heparin-induced thrombocytopenia (HIT), a Type II immune-mediated reaction where antibodies activate platelets, causing massive thrombus formation despite low platelet counts. HIT typically develops 5–10 days after heparin exposure. A platelet drop of ≥50% from baseline triggers suspicion. The nursing response is immediate: stop ALL heparin (including flushes and heparin-coated lines), notify the provider, and anticipate a non-heparin anticoagulant such as argatroban or bivalirudin. Never give platelets in HIT — they fuel the thrombotic process. Baseline and serial platelet counts (every 2–3 days) are essential monitoring during heparin therapy. Protamine sulfate is the reversal agent for UFH overdose, dosed at 1 mg per 100 units of heparin.

Key Distinctions

Don't confuse HIT with simple bleeding from over-anticoagulation — HIT causes clotting, not bleeding, despite low platelets. Students mix up monitoring labs: aPTT is for UFH, PT/INR is for warfarin, and anti-Xa is for LMWH. Protamine reverses heparin; vitamin K reverses warfarin — swapping these is a common exam trap.

Clinical Pearl

HIT = platelets drop but clots climb. Think "low count, high danger" — the threat is thrombosis, not hemorrhage. Stop every source of heparin, including line flushes.