It is possible that the lower viral load in the 3TC group at baseline resulted in a lower rate of virological failure and the detection of resistance mutation, however as the difference between the two groups was non-significant (p = 0.27) we would not expect this to significantly influence our results. A major limitation to our study concerns the unsystematic approach to resistance testing performed in failing patients. This may partly reflect the changing advice from clinical guidelines on resistance testing over time. As less than a fifth of patients check details had resistance tests

performed at the time of virological failure we may have underestimated the incidence of resistance mutation. Additionally, a significantly higher proportion of patients failing on 3TC had resistance tests performed compared to those failing on FTC containing regimens (19.1% v 14.0% (p = 0.03)) which may have selected towards a higher

frequency of resistance detection in the FTC group. In conclusion, although there is a trend towards increased risk of development of M184V or K65R resistance mutations in patients taking 3TC rather than FTC, this fails to reach significance. Overall we observed very low rates of virological failure in patients taking either regimen which suggests that other factors, including cost and patient acceptability, may become increasingly important when choosing between these agents. Steering Committee: Celia Aitken, Gartnavel General Hospital, Glasgow; David Asboe, Anton Pozniak, Chelsea & Westminster Hospital, London; Clare Booth, Royal Free NHS Trust, London; Patricia Cane, PLX-4720 research buy Health Protection RANTES Agency, Porton Down; Hannah Castro, David Dunn, David Dolling, Esther Fearnhill, Kholoud Porter, MRC Clinical Trials Unit, London; David Chadwick, South Tees Hospitals NHS Trust, Middlesbrough; Duncan Churchill, Brighton and Sussex University Hospitals NHS Trust; Duncan Clark, St Bartholomew’s and The London NHS Trust; Simon Collins, HIV i-Base, London; Valerie

Delpech, Health Protection Agency, Centre for Infections, London; Anna Maria Geretti, University of Liverpool; David Goldberg, Health Protection Scotland, Glasgow; Antony Hale, Leeds Teaching Hospitals NHS Trust; Stéphane Hué, University College London; Steve Kaye, Imperial College London; Paul Kellam, Wellcome Trust Sanger Institute & UCL Medical School; Linda Lazarus, Expert Advisory Group on AIDS Secretariat, Health Protection Agency, London; Andrew Leigh-Brown, University of Edinburgh; Nicola Mackie, Imperial NHS Trust; Chloe Orkin, St. Bartholomew’s Hospital, London; Philip Rice, St George’s Healthcare Trust, London; Deenan Pillay, Andrew Phillips, Caroline Sabin, University College London Medical School; Erasmus Smit, Health Protection Agency, Birmingham Heartlands Hospital; Kate Templeton, Royal Infirmary of Edinburgh; Peter Tilston, Manchester Royal Infirmary; William Tong, Guy’s and St.