Multicentre (Ottawa and Halifax) prospective cohort study using a diagnostic approach in patients clinically suspected to have HIT that combines pretest probability assessment with quantitative interpretation of anti-PF4 assay.

The proposed is a prospective cohort study exploring a novel diagnostic approach to Heparin Induced Thrombocytopenia (HIT) using a combination of pretest probability assessment and quantitative interpretation of the anti-platelet factor 4 Immunological assay (anti-PF4). Patient with a clinical suspicion of HIT will follow the study diagnostic algorithm (Figure 1). The study algorithm will be considered a safe approach to move forward into a larger RCT if the upper limit of the 95% confidence interval for 'false negative management failures' is ≤ 4% based on a Serotonin Release Assay (SRA) gold standard. The main objective of the pilot study is to inform feasibility and recruitment barriers for a larger randomized control trial.

Inclusion Criteria:

• Patient with clinical suspicion of HIT by treating physician

Exclusion Criteria:

1. Less than 18 years of age;
2. Prior diagnosis of HIT ever;
3. Patient enrolled within preceding 100 days;
4. Functional/ confirmatory platelet activation results available at the time of enrollment;
5. Requiring cardio-pulmonary bypass or percutaneous cardiac angioplasty or any other cardiac or vascular surgery/procedure requiring intra-operative/procedural heparin administration planned within 30 days;
6. Unable to complete study follow up;
7. Unable to obtain consent (or proxy consent from substitute decision maker where applicable);
8. Life expectancy less than 30 days;
9. Greater than 72 hours from clinical suspicion of HIT and/or request for HIT anti-PF4 ELISA testing.