Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. The authors declare the following other interests: ST-P is supported by a National Institute for Health Research (NIHR) career development fellowship (CDF-2016-09-018). JD is supported by the NIHR Birmingham Biomedical Research Centre at the University Hospitals Birmingham NHS Foundation Trust and the University of Birmingham (BRC-1215-20009). Support was also provided through UK Aid funding to The Foundation for Innovative New Diagnostics (FIND). The views expressed are those of the authors and not necessarily those of FIND, the National Health Service, the NIHR or the Department of Health and Social Care. Further details of The BMJ policy on financial interests is here: https://www.bmj.com/sites/default/files/attachments/resources/2016/03/16-current-bmj-education-coi-form.pdf.