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Could we, should we? Yes

Fox, Juliea; White, Peter J.b,c; Weber, Jonathand; Garnett, Geoff P.c; Ward, Helene; Fidler, Sarahc

doi: 10.1097/QAD.0b013e32834a1cbb

aDepartment of HIV, Faculty of Medicine, Guys and St Thomas’ NHS Trust/Kings College London

bModelling and Economics Unit, Health Protection Agency

cDepartment of Infectious Disease Epidemiology, MRC Centre for Outbreak Analysis and Modelling

dDepartment of Genitourinary Medicine and Infectious Disease

eDepartment of Infectious Disease Epidemiology, Faculty of Medicine, Imperial College London, London, UK.

Correspondence to Geoff P. Garnett, Department Infectious Disease Epidemiology, MRC Centre for Outbreak Analysis and Modelling, Imperial College London, London W2 1PG, UK. E-mail:

Received 18 May, 2011

Accepted 21 June, 2011

In their comment on our article describing the calculation of a risk exposure score [1], Gerberry and Blower [2] argue that the score only applies retrospectively, is unlikely to be calculable in the real world, and has so much uncertainty that it is unhelpful.

We disagree, first, like all studies of risk, the score is based on past studies, but the calculation can be applied both to recent behaviour and to explore how risk could be modified. This is the typical basis of risk counselling. Second, in our HIV clinics, many people in discordant partnerships do know the viral load and HIV stage of their partner. Third, the way that the variation based on uncertainty is presented by Gerberry and Blower [2] is misleading: taking the upper bounds of the 95% confidence interval (CI) for two parameters and using them together gives an upper 99.875% confidence bound, and similarly for the lower bounds. For three parameters, the bounds get even wider. Thus, ‘using the limits of the 95% CIs for each of the included risk factors’ creates extreme ranges. In our article, we presented 95% CIs for the overall uncertainty, as is standard practice.

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Conflicts of interest

There are no conflicts of interest.

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1. Fox J, White PJ, Weber J, Garnett GP, Ward H, Fidler S. Quantifying sexual exposure to HIV within an HIV-serodiscordant relationship: development of an algorithm. AIDS 2011; 25:1065–1082.
2. Gerberry DJ, Blower SM. Could we – should we – calculate a risk exposure score for an HIV-negative individual in a serodiscordant couple?. AIDS 2011; 25:1119–1121.
© 2011 Lippincott Williams & Wilkins, Inc.