Episode 017: Heme/Onc Emergencies, Pt. 6: Heparin-Induced Thrombocytopenia (HIT)

Emergencies happen in hematology and oncology. This is a fact. But how do we manage these emergencies? Look no further. In this episode, we talk all about our third hematologic emergency: heparin-induced thrombocytopenia (HIT)!

Be sure to check out episode 009 on thrombocytopenia for a general approach and differential!

HIT:
- Any time a patient with heparin exposure and now with a new thrombotic event, you need to think about HIT!

What is HIT?
- Type 1: a transient drop in platelets after heparin is started
- Type 2:
**The scary one! Antibody-mediated process
**Heparin molecules bind to platelet-factor 4 (PF4)
**This complex activates platelets, which then further releases more PF4 from the platelets

What is the difference between HIT and HITT?
- HITT is when there is also thrombosis (HIT + Clot)

Why is this more common in the cardiac ICU?
- It is believed that IgM interacts with ultra-long complexes, which heparin is
- Lots of heparin is required for cardiac surgery
- Therefore lots of exposure to heparin increases likelihood, increasing likelihood for IgM to IgG class-switching; HIT is IgG-mediated process
** Remember - since this is antibody-mediated, therefore it takes a few days for the antibodies to form in patient with a new diagnosis of HIT!

How to stratify?
4-T score (MDCalc Link: https://www.mdcalc.com/4ts-score-heparin-induced-thrombocytopenia)

Workup:
- Sent HIT ELISA test in patient with high suspicion
- ELISA just suggests if the HIT antibody is present
- If ELISA positive, then do confirmatory assay, i.e., is this antibody actually doing anything, is the "serotonin-release assay”
- Send 4 extremity dopplers to look for thrombosis
- STOP heparin/heparin-derived products and SWITCH anticoagulant, such as argatroban, fondaparinux, bivalirudin (do not wait for a positive test if your suspicion is high enough!)

If HIT positive:
- Add heparin to their allergy list
- Continue anticoagulation until platelets are recovered (>150K)
- Continue anticoagulation for 3-6 months for patients with HITT

Words of wisdom: If patient comes from outside hospital and starts having decreasing platelets, consider HIT in your differential!

References:
https://ashpublications.org/blood/article/119/10/2209/29530/How-I-treat-heparin-induced-thrombocytopenia- great review article from ASH on HIT

Please visit our website (TheFellowOnCall.com) for more information
Twitter: @TheFellowOnCall
Instagram: @TheFellowOnCall
Listen in on: Apple Podcast, Spotify, and Google Podcast