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Who is the patient?

HIT is a relatively common adverse event in certain populations who receive unfractionated heparin for 1 week or more (1) although it can occur with any form of heparin.

The frequency of HIT after exposure to heparin is very variable and is influenced by the heparin preparation duration, and the patient population (1):

Table: Risk factors for immune HIT  

Risk factor
High risk
Moderate-Low risk

Heparin duration

>4 days

 <4 days 

Type of heparin




Bovine heparin

Porcine heparin

Primary condition





Clinical symptoms of HIT occur in only a minority of patients who develop HIT antibodies and the proportion of patients with HIT antibodies who develop thrombocytopenia or thrombosis differs considerably in different clinical settings. For example:

  • In postoperative orthopaedic surgery patients given UFH prophylaxis, antibodies occur in about 15% of patients and the incidence of clinical HIT is about 5%.
  • After cardiac surgery the frequency of HIT antibody formation is high (about 50%) but the incidence of clinical HIT is about 2 to 3% (2).
  • The frequency of HIT is estimated to be between 0.1% and 1% in postsurgery patients given LMWH and medical/obstetric patients receiving LMWH who received UFH first(1).

The patient groups at the highest risk of HIT include postsurgery orthopaedic, cardiac and vascular surgery patients who are receiving UFH for 1-2 weeks and, likely, other postsurgery populations(1).

1. Warkentin TE, Greinacher A, Koster A, Lincoff AM. Treatment and prevention of heparin-induced thrombocytopenia: ACCP evidence based clinical practice guidelines (8th Edition). Chest 2008; 133: 340-380

2. Warkentin TE. New approaches to the diagnosis of Heparin-induced thrombocytopenia. Chest 2005; 127:35s-45s

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When to suspect HIT