Coagulation

Coagulation items almost always test which lab tracks which drug and what the therapeutic target is: a therapeutic INR of 2–3 for most warfarin clients (2.5–3.5 with mechanical mitral valves, per ACC/AHA), and an aPTT of 1.5–2.5 times the control for unfractionated heparin. The classic “tell” is a lab paired with a route — the answer often hinges on recognizing that low-molecular-weight heparin (enoxaparin) needs no routine aPTT monitoring (anti-Xa is used when monitoring is indicated), and that a sudden, unexplained >50% platelet drop 5–10 days into heparin signals heparin-induced thrombocytopenia (HIT), where you stop all heparin rather than transfuse platelets. Direct oral anticoagulants (apixaban, rivaroxaban) are reversed by andexanet alfa, and dabigatran by idarucizumab — not vitamin K.

The trap students fall into is confusing this with hemorrhage (you act on falling BP and a soaked dressing, not a lab) and with general monitoring/laboratory critical-value recall. A clean way to keep the heparin labs straight: aPTT’s extra letters go with unfractionated heparin, the drug that does need bedside titration.