title: The effect of participant nonresponse on HIV prevalence estimates in a population-based survey in two informal settlements in Nairobi city.
abstract: BACKGROUND
  Participant nonresponse in an HIV serosurvey can affect estimates of HIV prevalence. Nonresponse can arise from a participant's refusal to provide a blood sample or the failure to trace a sampled individual. In a serosurvey conducted by the African Population and Health Research Center and Kenya Medical Research Centre in the slums of Nairobi, 43% of sampled individuals did not provide a blood sample. This paper describes selective participation in the serosurvey and estimates bias in HIV prevalence figures.
  
  
  METHODS
  The paper uses data derived from an HIV serosurvey nested in an on-going demographic surveillance system. Nonresponse was assessed using logistic regression and multiple imputation methods to impute missing data for HIV status using a set of common variables available for all sampled participants.
  
  
  RESULTS
  Age, residence, high mobility, wealth, and ethnicity were independent predictors of a sampled individual not being contacted. Individuals aged 30-34 years, females, individuals from the Kikuyu and Kamba ethnicity, married participants, and residents of Viwandani were all less likely to accept HIV testing when contacted. Although men were less likely to be contacted, those found were more willing to be tested compared to females. The overall observed HIV prevalence was overestimated by 2%. The observed prevalence for male participants was underestimated by about 1% and that for females was overestimated by 3%. These differences were small and did not affect the overall estimate substantially as the observed estimates fell within the confidence limits of the corrected prevalence estimate.
  
  
  CONCLUSIONS
  Nonresponse in the HIV serosurvey in the two informal settlements was high, however, the effect on overall prevalence estimate was minimal.
authors.affiliation: African Population and Health Research Center, Shelter Afrique Centre, 2nd Floor, Longonot Road, Upper Hill P, O, Box 10787, 00100, Nairobi Kenya. abdhalah.ziraba@lshtm.ac.uk.
authors.forename: Abdhalah K
authors.initials: AK
authors.lastname: Ziraba
authors.forename: Nyovani J
authors.initials: NJ
authors.lastname: Madise
authors.forename: Mwau
authors.initials: M
authors.lastname: Matilu
authors.forename: Eliya
authors.initials: E
authors.lastname: Zulu
authors.forename: John
authors.initials: J
authors.lastname: Kebaso
authors.forename: Samoel
authors.initials: S
authors.lastname: Khamadi
authors.forename: Vincent
authors.initials: V
authors.lastname: Okoth
authors.forename: Alex C
authors.initials: AC
authors.lastname: Ezeh