HIT (Heparin Induced Thrombocytopaenia)

• Complication of UFH or LMWH
• An immune-mediated disorder
• Possible life-threatening venous & arterial thrombosis
• Identify early, stop heparin and substitute alternative anticoagulation

• Inform patients that HIT is a possible complication of heparin therapy
• Before starting heparin, check baseline platelet count
• Monitor platelet count in the clinical situations below

Patients receiving UFH/LMWH

Post-operative patients

• Include obstetric post-operative patients
• Check platelet count on alternate days
• start from day 4 until day 14 of heparin treatment or until heparin is stopped
• If received UFH in the previous 100 days and are now receiving UFH or LMWH
• start platelet count monitoring from day 2 of treatment

Patients not post-operative

• Incidence of HIT is <1%
• monitoring for HIT not necessary
• Investigate for HIT if:
• unexplained drop in platelet count >30% of baseline or
• development of new thrombosis while on UFH/LMWH or
• clinical feature of HIT

Clinical features of HIT

• A >30% fall in platelet count
• Extension of previous thrombus
• New arterial/venous thrombosis
• Thrombosis in an unusual site (cerebral, renal, skin necrosis)
• Acute systemic reaction after UFH IV bolus
• cardiorespiratory, neuralgic or unusual symptoms within 30 min
• Disseminated intravascular coagulation (DIC)
• Skin lesions at heparin injection sites
• Despite low platelet count, bleeding is uncommon

HIT suspected

• Perform pre-test probability scoring for HIT

Pre-test probability scoring

HIT antibody

High clinical suspicion (4T score ≥6)

• HIT antibody positive, HIT confirmed
• HIT antibody negative, possibility of HIT 3-16%
• continue alternative anticoagulants until further tests
• seek haematology advice

Intermediate clinical suspicion (4T score= 4–5)

• HIT antibody positive, HIT possible~60%
• continue alternative anticoagulants until further tests
• seek haematology advice
• HIT antibody negative, HIT unlikely

• Obtain blood sample for HIT antibody testing. Check sample requirement
• If HIT antibodies results are not immediately available, make treatment decisions on clinical grounds
• if in doubt, contact haematology

Intermediate or high pre-test probability of HIT

• Stop heparin
• Start alternative anticoagulant in treatment doses
• see Non-heparin anticoagulants
• Do not start warfarin
• Warfarin already started:
• omit further doses and
• give Vitamin K₁ (phytomenadione) 5 mg by slow IV injection
• while introducing alternative anticoagulation
• Platelet transfusion is relatively contraindicated
• thrombocytopenia is rarely severe
• not associated with bleeding

Patients with low pre-test probability

• Continue heparin
• Contact haematology consultant about HIT antibody testing

When

• Start warfarin only when:
• platelet count has recovered to >150 x 10⁹/L
• patient is fully anticoagulated with alternative anticoagulant
• Start warfarin using lower loading dose of 5 mg. See Warfarin initiation guideline

After starting warfarin

Patients on argatroban

• Continue argatroban
• for a minimum of 5 days and
• until INR >4 for 2 days
• once stopped, allow INR to revert to usual target range

Patients not on argatroban

• Continue alternative anticoagulant
• for a minimum of 5 days and
• until INR in target range for 2 consecutive days

Length of anticoagulation

• Ensure platelet count remains stable
• Give therapeutic anticoagulation for 3 months after HIT associated with a thrombotic complication and for 4 weeks following HIT without a thrombotic complication

• Document HIT in patient notes, electronic records

Inform GP and Patient

• Increased risk of thrombosis if given UFH or LMWH in the 100 days after HIT
• If patient requires anticoagulation with heparin after more than 100 days, seek advice from haematology consultant
• Document HIT in discharge letter
• Ask GP to monitor platelet count