A keen eye for risk.

A 56 year old woman was referred to the rapid access chest pain clinic with stable angina pectoris. Her only cardiovascular risk factor was a family history of atherosclerosis, with her mother suffering from myocardial infarction at 50. Her body mass index was 20 kg/m and the main abnormality on examination was the eye sign shown in . Fasting serum cholesterol and low density lipoprotein were both markedly raised at 12.2 mmol/L and 9.3 mmol/L, respectively. Triglyceride levels were relatively normal at 1.9 mmol/L. Given the history, examination, and lipid abnormality, what further blood test would confirm the underlying diagnosis?