Heparin-Induced Thrombocytopenia (HIT) (BSH 2012)
Type II HIT Pathophysiology
Heparin is a bioproduct produced from pig intestines --> why so many problems with antibody formation.
Danaparoid is a heparinoid also produced from pigs but a different fraction.
Exposure to heparin may --> Heparin-platelet factor 4 antibodies.
The antibody primarily recognises a heparin-induced conformational change in platelet factor 4. The type of change is affected by the length of the heparin molecule --> different incidence of HIT depending on the preparation.
IgG-Heparin-PF4 complexes activate platelets and thrombin --> prothrombotic state.
Bovine > Porcine heparin (all UK Heparin is porcine)
Surgical > medical patients
UFH > LMWH (5% vs 0.5% in orthopaedic prophylaxis)
Risk with prophylactic LMWH 0.2-0.9% depending on the study
Plt count falls 5-10 days after starting heparin
(more rapid if received heparin on a previous occasion in last 3 months)
Plt count falls >50%, nadir count <55 (<15 is unusual)
50% have associated thrombosis
Assess the re-test probability with 4T score.
If low risk, HIT excluded without need for further tests
If non-low risk, switch to alternative anticoagulant whilst tests performed
All patients due to have heparin should have a baseline platelet count
Post-op patients, inc obstetric, receiving UFH should have plt count every 2-3 days from days 4-14 or until heparin stopped.
Post-op cardiac bypass patients receiving LMWH should have plt count every 2-3 days from days 4-14 or until heparin stopped.
Do Not Monitor:
Post-op patients other than cardiac bypass who receive LMWH
Medical and obstetric patients receiving heparin
Clinical Manifestations of HIT
Common – DVT, PE, Arterial thrombosis (ACS, stroke, peripheral artery thrombosis)
Other – skin lesions, venous gangrene, adrenal bleed, total global amnesia, rigors, acute collapse and death, warfarin-induced skin necrosis.
Screening tests – Antigen Assays (2 methods)
Screening test looks for H-PF4 antibodies. 15% of patients exposed to heparin will produce H-PF4 antibodies but only a small fraction will lead to clinically significant HIT. i.e. negative test more useful than positive.
Chemiluminescent assay (HemosiL AcuStar) used at Addenbrookes.
Neg pred value >95%.
Pos pred value 70%
Start with magnetic particles coated with PF4 complexed with polyvinyl
Source of Anti-PF4/Heparin antibodies (Control, Patient) is added
After incubation, the sample is washed and a tracer AHG antibody added
After 2nd incubation, a trigger for luminescent reaction is added
Luminescnce is instantaneous, producing a flash of light, detected by analyser.
Used at Bristol.
Similarly good neg pred value, weak positive pred value
Heparin-induced platelet aggregation assay.
Processed in Germany.
A washed platelet assay that tests platelet aggregation in different concentrations of heparin
Serotonin-release assay is a commercial test.
Requires a fresh platelet sample, radioactive serotonin and a healthy control patient.
Licensed – argatroban & danaparoid
Unlincensed but used – fondaparinux & bivalirudin
Basic principle is stop heparin and start alternative anticoagulation
Initial anticoagulant dose should be the same whether or not there is already thrombosis at the time of diagnosis.
Warfarin should be introduced once plt count is in the normal range.
It should be overlapped with original AC by 5 days using low-dose dosing.
Argatroban affects the INR and this should be >4 for 2 days before stopping
Bleeding is rare in HIT.
Platelet transfusion can be used safely if bleeding occurs, but should not be used prophylactically.
Heparinoid that indirectly inhibits Xa and thrombin
Predictable dose response and long half-life (24 hours)
Monitor with a specific anti-Xa assay if wt >90kg or renal impairment
Given as bolus dose followed by reducing dose infusion regimen.
Direct thrombin inhibitor
Metabolised by liver
2.0 ug/kg/min to achieve APTT ratio of 1.5-3.0 (start at 0.5ug/kg/min if unwell)
5, 7.5 and 10mg based on weight of <50, 50-100 or >100kg
UFH and LMWH should be avoided in the future (but recurrence is rare)
Exception for cardiac surgery if >100 days since HIT and antibody test now neg.
N.B. Type I HIT
Occurs 2 days after starting heparin, results from direct effect of platelet activation by heparin. Will resolve rapidly and spontaneously whilst heparin continues to be given.