Hematology · USMLE Step 1 and Step 2

Heparin - Unfractionated & LMWH song

Heparin, LMWH, and HIT, as a song. Hear the short teaser here. The full song is on YouTube and Spotify.

$Heparin - Unfractionated & LMWH cover art$

Heparin - Unfractionated & LMWH

Full song on YouTube ↗Spotify ↗

Key points in this song10

Unfractionated heparin (UFH) binds antithrombin III, potentiating inhibition of thrombin (IIa) and factor Xa; LMWH primarily inhibits factor Xa with minimal effect on thrombin.
UFH is monitored with aPTT (therapeutic goal 60–100 seconds); LMWH does not require routine monitoring but anti-Xa levels are used in renal impairment, obesity, and pregnancy.
UFH is preferred over LMWH in renal failure (CrCl <30 mL/min) because it is not renally cleared, while LMWH accumulates and increases bleeding risk.
Heparin-induced thrombocytopenia (HIT) is a prothrombotic, immune-mediated condition caused by IgG antibodies against the heparin-PF4 complex, typically occurring 5–10 days after initiation; treatment requires immediate cessation of all heparin and initiation of a direct thrombin inhibitor (argatroban or bivalirudin).
The 4T score (Thrombocytopenia, Timing, Thrombosis, oTher causes) is used to assess pretest probability of HIT before confirmatory testing with serotonin release assay (SRA) or ELISA.
Protamine sulfate reverses UFH completely and LMWH partially (neutralizes the anti-IIa component but only ~60% of anti-Xa activity).
UFH is the anticoagulant of choice during pregnancy for acute indications requiring rapid reversal (e.g., peripartum), but LMWH is preferred for outpatient anticoagulation in pregnancy due to predictable dosing and lower risk of osteoporosis and HIT.
LMWH has greater bioavailability, longer half-life, and more predictable pharmacokinetics than UFH due to less binding to plasma proteins and endothelial cells.
UFH activates lipoprotein lipase, causing a transient decrease in triglycerides and increase in free fatty acids — a classic pharmacologic side effect.
Both UFH and LMWH do not cross the placenta and are safe for fetal exposure; warfarin and DOACs are contraindicated in pregnancy due to teratogenicity.