[1] Univ Oxford, John Radcliffe Hosp, Nuffield Dept Clin Lab Sci, Oxford OX3 9DU, England
[2] Univ Oxford, Nuffield Dept Med, Oxford OX3 9DU, England
[3] Coll Med, Blantyre, Malawi
[4] Malawi Liverpool Wellcome Programme Clin Trop Res, Blantyre, Malawi
[5] Univ Liverpool, Liverpool Sch Trop Med, Liverpool L3 5QA, Merseyside, England
[6] London Sch Hyg & Trop Med, London WC1, England