Clinical Relevance:

A rare but serious risk from heparin treatment is heparin induced thrombocytopenia (HIT).   There are two forms of HIT.

Type 1 patients have a transient decrease in platelet count without any further symptoms.

Type 2 HIT develops in up to 3% of patients on unfractionated (UFH) and in <1% of patients on low molecular weight heparin LMWH and causes thrombosis in up to 50% of these patients.

Type 2 HIT is due to an immune reaction with antibodies formed against a complex of platelet factor 4 (PF4) and heparin. These antibodies are not uncommon but only cause clinical problems when they lead to platelet activation (thrombotic risk) and consumption (thrombocytopenia).

Reference Range:

Please refer to CG18 – Heparin Induced Thrombocytopenia guideline on the intranet.

Results will be provided as Negative or Positive for the presence of PF4 antibodies and should be used in conjunction with an Intermediate or High pre-test probability score (4T).

Min. Volume: The volume of blood in coagulation samples must lie within the volume range as indicated by the size of the black fill line present on the tube.
Turnaround Time: Urgent: 1 hour
Age of Sample: Samples will be rejected if received more than 2 hours after venepuncture.
Specimen Requirements:
  • 1 red top (serum clot activator) sample.
  • All coagulation tubes must be adequately filled (see above).
  • All coagulation tubes must be mixed several times by gentle inversion immediately after venepuncture. Mixing the sample with the anticoagulant stops the sample clotting within the tube.
  • All HIT requests must be approved by a Haematology SpR or a Consultant Haematologist prior to the sample being taken and sent to the laboratory.
Limitations:
  • It is not possible to provide results on haemolysed, or lipaemic samples. These will be rejected with the appropriate comment.
  • Sample tubes that have expired cannot be accepted.
Analysing Laboratory: Coagulation (JCUH)
Referral Laboratory: Positive results will be verified at the Coagulation laboratory at the Royal Victoria Infirmary in Newcastle.