The inability to retain enrolled participants poses serious threats to both the internal and the external validity of a research study. Retention may be related to different factors depending upon the population, e.g. young age and longer trial duration 1, self-identifying as homosexual and having been a male sex worker in the past 6 months 2, and male sex, age <35 years, advanced HIV/AIDS disease and increasing malnutrition 3. As the HIV epidemic is focused in sub-Saharan Africa, antiretroviral programs as well as clinical trials necessitate a close examination of retention in African populations 3. Understanding factors associated with retention can help in informing future screening processes and plans for retention strategies.

The Kisumu Incidence Cohort Study (KICoS) was initiated in January 2007 to prepare the Kenya Medical Research Institute (KEMRI)/Centers for Disease Control and Prevention (CDC) site in Kisumu for participation in future efficacy trials of biomedical interventions to prevent HIV infection. Participants were enrolled and followed every 3 months for a total of 12 months. The purpose of this secondary analysis from KICoS was to determine 1) the proportion of participants attending all study visits and 2) demographic and behavioral factors associated with missing ≥ 1 visit.

Data from the adults and minors were combined for analyses. Bivariate and multivariate logistic regression analyses were used to determine demographic and behavioral factors associated with missing ≥ 1 visit. All of the independent variables were entered into the multiple logistic regression model. Independent variables were gender, ethnic group, level of education, age, religion, employment status, reminder of visit, main motivation for participating, migration history, marital status, lifetime number of sex partners, any drug use in the last three months, and any alcohol use in the last three months. The proportion of participants who chose the option “refuse to answer” to our questions of interest was less than 3%, so these responses were not included in the study analysis. All analyses were carried out using SAS for Windows version 9.2 (SAS, Cary, North Carolina, USA).

A total of 1,724 persons were pre-screened for the study (adults: 1,277; minors: 447). Of those, 1,106 completed screening (adults: 846; minors: 260) and 831 were enrolled (adults: 625; minors: 206); 46.4% were females. Of those enrolled, 74.3% were single, never married, separated, divorced or widowed (Table 1). Overall participant retention was 90% for the 12 months of follow-up.

Ninety percent of over 800 cohort study participants in Kisumu, Kenya completed all study visits over a 12-month period. This high rate of retention may be attributable in part to the collection of detailed contact and locator information of participants that enabled staff to contact participants, as well as the flexibility of the study design that enabled many participants to honor their visits within defined visit “windows”. It may also be due to the efficient clinic process. It has been noted that dropouts occur when participants’ perceived time and effort invested outweigh the perceived benefits of being in a study 6. It is of note that the visit reminder administered to a random half of participants six weeks before the scheduled visit was not significantly associated with completion of all visits. This could be because the lead time was too long; because we did not distinguish between telephone, in-person or mailed reminders, some of which may have been more effective than others; or, because implementation of the visit reminders was not documented and thus, could not be examined.

Our results showed that two demographic and no behavioral factors were associated with missing ≥ 1 study visit. Females had nearly three times the odds of not completing all study visits than men. This is concerning since the prevalence of HIV infection in women in this area of western Kenya is approximately twice that of men 7 and female-focused prevention trials are planned. Little research has been conducted on factors associated with missed visits in HIV prevention research, however, in studies of medication adherence, some studies have shown women to be less adherent (e.g., medication for heart failure 8, ART in India 9). Reasons women may have poor medication adherence are varied, representing their competing demands that include caring for children 10 and family, transportation costs and work commitments 11.

In addition, in our study, persons with more education (above secondary school) had higher odds of missing ≥ 1 study visit. Even though work was not associated with ≥ 1 missed visit, it may be that persons with more education would have a higher likelihood of being engaged in types of work which constrain schedules than those with less education and thus miss visits due to work requirements. In a study of loss to follow- up in an HIV treatment program in western Kenya, work commitment was a reason why men were lost to follow-up 10.

Our study had several potential limitations. First, our participants were volunteers and were recruited using convenience sampling, so they may not be a true representation of the Kisumu population. Second, we did not collect data on reasons for missed visits or type of employment. Third, data was not available on the type of reminder (via telephone, in-person, mailed) provided to half of the participants and the frequency of successfully reaching the participant using the methods. Finally, all of the relevant and important variables may not have been captured in our survey (e.g., peer or family social support, lack of transport or other barriers to attending visits).

In conclusion, retention was relatively high in this HIV prevention cohort study. Some amount of attrition is unavoidable as participation is voluntary and unpredictable events may require participants to miss a visit or leave the study. It is concerning, however, that among the groups more apt to miss ≥ 1 study visit was females. Females are affected disproportionately by HIV in this area of Kenya 12 and their full participation in HIV prevention studies is needed to assure that any intervention developed will work for them 13. For future HIV prevention studies in Kisumu, it may be beneficial to tailor retention strategies for women.