[1] Natl Inst Biol Stand & Controls, Div Haematol, Potters Bar EN6 3QG, Herts, England
[2] UK NEQAS Blood Coagulat, Sheffield, S Yorkshire, England
[3] Royal Hallamshire Hosp, Sheffield Haemophilia & Trombosis Ctr, Sheffield S10 2JF, S Yorkshire, England