Chapter 59: Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia is a prothrombotic disorder mediated by IgG antibodies that bind to conformational epitopes on PF4 when it is complexed with heparin. Typically, the platelet counts are only moderately reduced. Occasionally patients do not have thrombocytopenia, but their platelet counts decrease by 50% from pretreatment levels

Kelton JG et al. N Engl J Med 2013;368:737–744

Epidemiology

1. Frequency: Determined by heparin type (UFH > LMWH), patient type (surgery > medical), patient gender (F > M); odds ratio 17.4 for postoperative thromboprophylaxis (UFH > LMWH), and duration of heparin therapy (10 –14-day course >1 day: >2% vs. 0.02%)

   1. >1%: Postoperative patients: UFH at prophylactic or therapeutic dose (1–5%); cardiac surgery (1–3%)

   2. 0.1–1%: Postoperative patients: UFH flushes or LMWH in prophylactic or therapeutic dosages (0.1–1%); Medical patients: cancer (1%), UFH at prophylactic or therapeutic dose (0.1–1%), LMWH at prophylactic or therapeutic dose (0.6%), intensive care patients (0.4%)

   3. <0.1%: UFH flushes, obstetrics patients (non-surgical)

2. Thrombotic risk: Isolated HIT (untreated): 30-day cumulative rate ~50% after stopping heparin; initial rate of thrombosis per day over the first 1–2 days after discontinuing heparin: 5–10%

3. Mortality (9 studies, 876 patients): Overall population, ~20%; among patients with thrombosis, ~46%

Lee DH, Warkentin TE. Frequency of heparin-induced thrombocytopenia. In: Warkentin TE, Greinacher A, eds. Heparin-Induced Thrombocytopenia. 4th ed. New York, NY: Informa Healthcare USA; 2007:67–116

Linkins L-A et al. Chest 2012;141(Suppl 2):e495S–e530S

Warkentin TE. Clinical picture of heparin-induced thrombocytopenia. In: Warkentin TE, Greinacher A, eds. Heparin-Induced Thrombocytopenia. 4th ed. New York, NY: Informa Healthcare USA; 2007:21–66

Warkentin TE et al. Chest 2008;133(Suppl 6):340S–380S

Work-up

The diagnosis of HIT is based on both clinical and serologic findings and thus should be considered a clinicopathologic syndrome (Warkentin TE et al. Thromb Haemost 1998;79:1–7)

A. The work-up section for HIT outlines an approach for making or excluding a diagnosis of HIT and when to consider the initiation of alternative anticoagulation therapy for heparin/low-molecular-weight heparin that incorporates the following considerations:

1. Clinical suspicion (determining the magnitude and pattern of platelet count fall correlated with the clinical setting, presence or absence of arterial/venous thromboembolic events and presence of other HIT manifestations). The authors recommend a review of the peripheral blood smear to exclude pseudothrombocytopenia and assess for other abnormalities (eg, TTP) that may influence the differential consideration for HIT (see Table B)

2. Pretest Probability (determining the likelihood of HIT with 4Ts score; see Table C)

3. HIT laboratory testing (see Table D)

4. Posttest Probability (determining the likelihood of HIT that incorporates results of the pretest probability with the strength of the quantitative value from HIT testing; when indicated and where clinically feasible, additional testing for the presence of HIT antibodies with a washed-platelet functional assay) (see Table B)

5. Other considerations that have a bearing on HIT management/treatment:

   1. Weight of the patient in kilograms

   2. Determination of the patient's renal and hepatic status

   3. Presence or absence of any clinical bleeding

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