[1] John Radcliffe Hosp, NHS Blood & Transplant Unit, Oxford, England
[2] Univ Nottingham, Div Epidemiol, Nottingham NG7 2RD, England
[3] Royal Infirm, Dept Gastroenterol, Newcastle Upon Tyne, Tyne & Wear, England
[4] John Radcliffe Hosp, Dept Gastroenterol, Oxford, England
[5] NHS Blood & Transplant, Oxford, England
[6] Western Gen Hosp, GI Unit, Edinburgh EH4 2XU, Midlothian, Scotland