ORIGINAL RESEARCH ARTICLE Unlinked anonymous HIV prevalence among New Zealand sexual health clinic attenders: 2005– 2006 S M McAllister RN MPH* , N P Dickson FRACP FAFPHM*, K Sharples MSc PhD*, M R Reid MPH FAChSHM † , J M Morgan MRCP FAChSHM ‡ , E J MacDonald MBChB FAChSHM § , E Coughlan MBChB FAChSHM**, T M Johnston RN BA (Nursing) †† , N A Tanner DCH Dip Ven FRNZCGP ‡‡ and C Paul MB PhD* *Department of Preventive and Social Medicine, University of Otago Medical School, PO Box 913, Dunedin, New Zealand; † Auckland Sexual Health service, Auckland; ‡ Sexual Health Service, Waikato District Health Board, Hamilton; § Wellington Sexual Health Clinic, Wellington; **Chonk Church Sexual Health Centre, Christ Church; †† Sexual Health Clinic, Mid Central Health, Palmerston North; ‡‡ Sexual Health Clinic, Tauranga Hospital, Tauranga, New Zealand Summary: This unlinked anonymous study aimed at determining the prevalence of HIV among sexual health clinic attenders having blood samples taken for syphilis and/or hepatitis B serology in six major New Zealand cities over a 12-month period in 2005–2006. Overall, seroprevalence was five per 1000 (47/9439). Among men who have sex with men (MSM), the overall prevalence and that of previously undiagnosed HIV were 44.1 and 20.1 per 1000, respectively. In heterosexual men, the overall prevalence was 1.2 per 1000 and in women 1.4 per 1000. HIV remains to be concentrated among homosexual and bisexual men. Comparison with a previous survey in 1996–1997 suggests an increase in the prevalence of undiagnosed HIV among MSM and also an increase in the number of MSM attending sexual health clinics. The low prevalence of HIV among heterosexuals suggests no extensive spread into the groups identified at risk of other sexually transmitted infections. Keywords: HIV seroprevalence, unlinked, anonymous, sexual health clinics INTRODUCTION In New Zealand, as in many developed countries, the preva- lence of HIV infection in the general population remains to be very low. Hence, to detect early changes in the pattern of spread it is helpful in studying sentinel populations such as people attending sexual health clinics who are likely to have been practising sexual behaviours that put them at increased risk of HIV infection. 1,2 This also enables monitoring of HIV in subgroups whose behaviour puts them at higher risk of sexual transmission. Surveillance of HIV based on named testing can give a misleading picture as people who agree to be tested generally have a lower prevalence than those who decline. 3–5 For this reason, the use of unlinked anonymous testing, in which blood collected for another purpose is anonymized and unlinked before testing, is recommended by the World Health Organization for public health surveillance of HIV infection. 6 The New Zealand AIDS Epidemiology Group (AEG) has carried out two unlinked anonymous prevalence surveys of HIV infection in sexual health clinics in 1991–1992 7 and 1996–1997 (unpublished). These studies confirmed that the most affected group was men who have sex with men (MSM), and that prevalence among heterosexual men and women was low ( ,0.1%). Since the second survey in 1996– 1997, the survival of people with HIV had improved. In addition, from 2000 to 2006 there has been an increase in the annual number of people being diagnosed with HIV in New Zealand. 8 Together these changes would have resulted in an increase in the prevalence of diagnosed HIV. Since 2000, most of the increase in MSM diagnosed with HIV has been among those infected in New Zealand, while that among heterosexual men and women has been among people infected in countries where HIV prevalence is high. 9 A repeat unlinked anonymous study was undertaken during 2005–2006. The aims were to (a) determine the current preva- lence of HIV among attenders at sexual health clinics in New Zealand who had blood taken for syphilis and/or hepatitis B serology; (b) compare this with the two previous similar studies; and (c) estimate the proportion of HIV infections not being diagnosed by voluntary named testing. METHODS The study population involved new clients presenting at New Zealand public sexual health clinics in Auckland, Hamilton, Tauranga, Palmerston North, Wellington and Christchurch over a 12-month period in 2005–2006, having blood taken for syphilis and/or hepatitis B serology. Clients who returned for a repeat visit in the same clinic within the study period were included only once. Those who attended solely for an HIV test were excluded as they would not be considered part of a usual sexually transmitted disease clinic population. Comparison data of the baseline population, which included all sexual health clinic attenders, both new and follow-up visits, for Correspondence to: Dr Nigel Dickson Email: nigel.dickson@otago.ac.nz International Journal of STD & AIDS 2008; 19: 752–757. DOI: 10.1258/ijsa.2008.008153