The above letter was sent to Llewelyn et al.: Dr Llewelyn and colleagues offered the following reply

Charlotte A. Llewelyn PhD

National Blood Service–Cambridge Center, Long Road, Cambridge CB2 2PT, UK

Lorna M. Williamson MD

National Blood Service–Cambridge Center, Long Road, Cambridge CB2 2PT, UK

Michael F. Murphy MD

National Blood Service–Oxford Center, Oxford, UK

Audrey A.M. Todd MD

Royal Alexander Hospital, Paisley, UK

Rod Taylor PhD

University of Birmingham, Birmingham, UK