HIT. Gradually, evidence accumulated on the exact underlying mechanism. In 1984–1986, John G. Kelton and colleagues at McMaster University Medical School developed the laboratory tests that could be used to confirm or exclude heparin-induced thrombocytopenia. Treatment was initially limited to aspirin and warfarin, but the 1990s saw the introduction of a number of agents that could provide anticoagulation without a risk of recurrent HIT. Older terminology distinguishes between two forms of heparin-induced thrombocytopenia: type 1 (mild, nonimmune mediated and self-limiting fall in platelet count) and type 2, the form described above. Currently, the term HIT is used without a modifier to describe the immune-mediated severe form. In 2021 a condition resembling HIT but without heparin exposure was described to explain unusual post-vaccination embolic and thrombotic events after the Oxford–AstraZeneca COVID-19 vaccine. It is a rare adverse event (1:1 million to 1:100,000) resulting from COVID-19 vaccines (particularly adenoviral vector vaccines). This is also known as Thrombosis with Thrombocytopenia Syndrome or TTS. References External links Coagulopathies