Main Research Question:

Can two new types of test, one called the 4T's score and the other called a rapid assay, help doctors correctly identify which patients are unlikely to have heparin-induced thrombocytopenia (HIT)?

HIT is a severe allergic reaction to the blood thinner heparin. This allergic reaction can lead to heart attacks, strokes, limb amputations, and death. Because heparin is one of the most commonly used drugs in the hospital setting, it is very important that the investigators are able to correctly identify who can safely continue to take heparin and who cannot.

It can be very difficult to diagnose HIT because it can look like many other medical conditions and the best laboratory tests for HIT are difficult to run and only available at specialized centres.

It would be very helpful if doctors had tests they could use that would tell them quickly and accurately which patients with symptoms that look like HIT really do have HIT (and require urgent treatment with another type of blood thinner) and which patients are very unlikely to have HIT (and could continue to take heparin safely). In this study, the investigators will compare the 4T's score (a scoring system that assigns "points" to the presence or absence of specific clinical features) and a rapid laboratory test with the old laboratory test to find out if one or both of these types of tests are useful for telling doctors which patients have HIT and which patients don't have HIT.

Inclusion Criteria:

• Suspected heparin-induced thrombocytopenia

Exclusion Criteria:

• Unable to give consent
• Unable to provide a blood sample
• Previous entered in study