Heparin induced thrombocytopenia

 Mortality- untreated HIT 20%

Autoantibody against heparin plus PF4 receptor on platelets ( similar to ternary mechanism in drug induced hemolytic anemia). Clinically significant type of HIT is type II.  Type I is a transient drop of plt within 2 days of heparin exposure. Type I is not clinically significant.

4T score calculator

1. If low probability--ct heparin, look for other causes. If OD 0.6 or less, this rules out HIT.

2. 4T score 4-5--> intermediate probability--> stop heparin. start non heparin anticoagulant. Send PF4 antibodies and SRA. If OD ≥2.00, consider the diagnosis of HIT to be confirmed per uptodate authors. If PF4 OD is between 0.6 and 1.99, send functional assay.

3. If a functional assay is not readily available, we use an OD >1.00 as confirmation of HIT in patients with an intermediate or high 4Ts score.

4.  4T 6-8 and an OD ≥1.50,  HIT confirmed; if the OD is between 0.60 and 1.49, we obtain a functional assay

Treatment

1. Discontinue heparin. Get US of all 4 extremities. If thrombosis documented, will need 3 months full dose anticoagulation, if no thrombosis, rx of heparin induced thrombocytopenia is treated for 30 days.

2. There are good data to support use of DOAC- Rivaroxaban, Apixaban, in the use of acute HIT without a preceding parenteral agent. For edoxaban and dabigatran, as was used in studies 5 days of parenteral anticoagulant is recommended in acute thrombosis.

3. Not much data to support argatroban in the treatment of acute HIT. Can use in pts going to surgery or who may need immediate reversal. Avoid argatroban in liver dysfunction. Argatroban may be preferred in kidney issues, although apixaban has been used.

4. Initiation of DOAC does not require platelet recovery. Transition when patient is clinically stable. No overlap. If pt is already on a parenteral agent such as argatroban or bivalirudin, start DOAC within 2 hours of stopping argatroban or bivalirudin infusion. DOAC start within 8-12 hours after stopping danaparoid infusion, and 24 hours after last dose of fondaparinux.

5. Pregnancy: fondaparinux The DOACs are contraindicated in individuals with a mechanical heart valve or during pregnancy or breastfeeding.

6. Role of IVIG: Individuals with platelet activation and thrombocytopenia from HIT-like antibodies in the absence of heparin exposure may need additional interventions to block platelet activation