Patients admitted to hospital with chest pain due to reduced blood flow to heart muscle (diagnosis Acute Coronary Syndrome) can be treated with medication and an angioplasty ± stent procedure, which restores blood flow to the heart. Antiplatelet drugs (Aspirin and Clopidogrel) are blood thinning treatments and research has reported they reduce heart attacks, death and stroke. The investigators know some patients do not respond fully to Clopidogrel but currently patients are not tested for this.