Ziraba et al. BMC Public Health 2011, 11:663 http://www.biomedcentral.com/1471-2458/11/663 Nairobi province, had ever been tested for HIV and received results [8]. In a later national survey in 2008, 73.8% of women and 57.3% of men in Nairobi province had ever tested and got results [10]. It is possible that there are variations in testing coverage within the slums just like it has been observed for other health indicators [11], however no study has been carried yet. Globally, there has been an ongoing debate on the need to increase testing rates. In 2007, WHO and UNAIDS published guidelines for provider-initiated testing and counselling (PITC), to compliment the traditional client- initiated testing and counselling [12]. With increasing availability of relatively effective interventions such as pre- vention of mother-to-child transmission (PMTCT) and anti-retroviral therapy (ART), as incentives, provider- initiated testing and counselling is being promoted as a way of increasing testing coverage in several countries, including Kenya which published the first guidelines in 2008 for this purpose [13]. In the PITC model, individuals presenting to a health facility are encouraged to be tested for HIV as part of the routine medical investigation, but are free to opt-out of the testing if they so wish. While Kenya is among the countries with relatively high coverage of HIV testing (37%) in sub-Saharan Africa, this is still low [14]. One of the most likely explanations for the poor uptake of HIV/AIDS testing services in Kenya and many developing countries is related to the high levels of stigma associated with HIV/AIDS[15-17]. Consequently, many people continue to die without knowing their status and without accessing HIV/AIDS treatment, HIV positive mothers continue to give birth without using PMTCT ser- vices [18] and many discordant couples are not aware of partner’s HIV status to enable them to take action [19-21]. Those who seek testing and counselling tend to do it late when they are symptomatic or perceive themselves to be at a high risk of HIV infection [22-26]. Socio-demographic characteristics such as gender (i.e. females), young age, higher education, and higher socioeconomic status have been reported to be associated with use of testing and counselling, but no study has been carried in the informal settlements [27-29]. Clinic-based studies on testing behaviour, which are the commonest, fail to capture views and characteristics of those who do not go for testing. Secondly, prior stu- dies have not discerned predictors for CITC from those of PITC and yet these are likely to be different. This study uses data from testers and non-testers to examine prior HIV testing status, identify unmet need for testing, and predictors of HIV testing. We hope that our results will help inform HIV prevention and care programs to adjust their activities to reach people who do not know their HIV status and hopefully reduce risk of new infec- tions, and promote early initiation of treatment among those who are infected. Page 2 of 10 Methods We used data from a cross-sectional population-based sero-survey nested in a longitudinal framework (Nairobi Urban Health and Demographic Surveillance System- NUHDSS) run by the African Population and Health Research Center (APHRC). The sero-survey was imple- mented in collaboration with the Kenya Medical Research Institute (KEMRI). The survey was conducted in two slums; Viwandani and Korogocho between 2006 and 2007. A simple random sample of 5,004 respondents estimated to be adequate to answer the key research questions was drawn from the NUHDSS database of over 60,000 indivi- duals for testing. After updating the residence status in a subsequent NUHDSS data collection round, 237 of those sampled were confirmed to have been non-residents at the time of sampling and were dropped from the final sample. Thus the effective sample was 4,767 (5,004 minus 237). Out of the sample of 4,767, 3,162 individuals (66.3%) con- sented to be interviewed, 2,721 (57.1%) provided a blood sample while 2,590 (54.3%) provided both interview and a blood sample. Participants had the option of accepting the interview only, the blood test only, accepting both or refusing both. Participation in the research was limited to females aged 15-49 years and males 15-54 years. We took a higher upper age limit for males because HIV prevalence among males tend to peak at a later age compared to women. Trained counsellors and phlebotomists inter- viewed respondents and drew samples respectively. The study received ethical approval from the Kenya Medical Research Institute’s Scientific and Ethical Review commit- tees. Proper consent procedures were observed for all participants prior to participation. Data collection and Measurements A list of all sampled clients was generated with enough information to enable field workers to positively identify the respondents. On the other hand, the questionnaire and blood sample filter papers did not contain any identi- fiers except a new ID that enabled linkage to the NUHDSS data. Interviews were conducted using a pre-tested and structured questionnaire. For individuals who had had more than one testing event in the past, data presented here only refers to the latest test event. Blood sample collection involved aseptically collecting drops of blood from a finger prick using diabetic pen- devices with non-reusable lancets. For each participant, 5 drops of blood were collected and air dried on a filter- paper card. Each dried blood spot (DBS) sample was labelled with a unique identification number for enabling creation of a link between the sero-data, survey data and the individual demographic surveillance data from the NUHDSS database. Participants who wanted to know their HIV status were referred to our collaborating VCT providers for counselling and testing services. HIV