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

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

The Fellow on Call: The Heme/Onc Podcast