Heparin Platelet Factor 4 (HIT) Assay

Methodology: 
Enzyme-Linked Immunosorbent Assay (ELISA)
Performed: 
Mondays, Wednesdays and Fridays
Reported: 
Within 5 days
Use: 
This assay is used in the diagnosis of the heparin-induced thrombocytopenia, which is an adverse drug reaction to heparin due to IgG antibodies to complexes of heparin and platelet factor 4. HIT is associated with platelet activation, venous and arterial thrombosis and a moderate thrombocytopenia. It is not associated with bleeding.
Specimen Requirements: 
One 4 mL light blue-cap tube (3.2% sodium citrate) or a clotted blood tube (a red- or gold-top serum separator tube)
Reference Values: 
  • Negative: < 0.4 optical density (OD) units
  • Positive: > 0.4 OD units

The assay detects IgG and IgM antibodies to a complex of heparin and the platelet protein termed platelet factor 4 (PF4), a strong, positively charged tetrameric protein derived from the alpha-granules of platelets. PF4 binds
strongly with heparin. Heparin-PF4 binding induces a conformational change in PF4 exposing certain cryptic epitope(s), to which the IgG antibody binds.

  1. HIT should be suspected clinically in a patient who is experiencing suspicious clinical features and who received heparin 5 - 10 days previously. This time can be reduced to several hours when heparin is given to a patient who has previously received the drug in the past 120 days.
  2. HIT should be suspected when the platelet count has decreased by 50% or more below baseline values or has decreased to less than 150,000 with a minimal platelet decrease of 30,000.
  3. In HIT, the platelet nadir is typically ≥ 20,000/μL.

The HIT antibody assay should be requested if and only if there is a clinical suspicion of heparin-induced thrombocytopenia. A positive antibody assay is frequently encountered in asymptomatic patients who have recently received high doses of unfractionated heparin.

Unacceptable Conditions:
  • Ambient
  • Specimens greater than 48 hours old
Stability (collection to initiation of testing):
  • Ambient: Unacceptable
  • Refrigerated: Up to 48 hours at 2 - 8°C
  • Frozen: Preferred
CPT Code (s): 
86022
Notes: 

UFHPL Test #: 60070

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