gms | German Medical Science

Background: Large-scale population-based cohort studies allowing for longitudinal medical analyses are crucial for epidemiological research, but they are often challenging and costly to follow-up [1]. In order to gain valuable insights into the health-to-disease transition, reliable and comprehensive follow-up information on health effects is needed, but the response rates are often low [2]. This project aims to describe the design and methodology of a clinical end point follow-up assessment in the Hamburg City Health Study (HCHS), and to identify response patterns.

Methods: The HCHS is an ongoing population-based cohort study including randomly selected inhabitants of Hamburg aged 45-74 years, and focuses on obtaining substantial knowledge about important risk and prognostic factors in major chronic diseases. Six years after the first enrollment in 2016, a census follow-up questionnaire was sent out in order to assess the participants´ vital status and information about their health conditions, including the onset of 64 diseases highly relevant to public health. The questionnaire was available online as well as in hard copy format. After two postal reminders, telephone interviews were conducted to increase the response rate. Mortality and causes of death were systematically identified via death records and forensic documents. Medical records were used to adjudicate a prespecified selection of clinical end points.

Results: From 16,065 participants included in this census follow-up, 15,707 received the clinical end point questionnaire, and 14,240 reported valid information, leading to a response rate of 90.7%. Postal reminders and the offer to participate via paper format or telephone interview increased the response rate significantly (42.7 percentage points). Most of the participants (77.6%) used the online version of the questionnaire, 15.7% preferred the paper format, and 6.5% responded via telephone. Participants who took part in the interview were more likely to have prevalent chronic disease (p < 0.001) compared to those who responded online. Those who used the hard copy questionnaire were significantly older (median age: 70 [interquartile range 60-77] years, p < 0.001) and more often female (60.1%, p < 0.001). The responders differed from the non-responders in several aspects: Participants who refused to participate were older and more likely to show prevalent disease while those who could not be reached were younger compared to participants who took actively part in the follow-up (p < 0.001).

Clinical end point adjudication based on medial records was performed in several hundred cases and we identified false-positives as well as false-negatives. The results in terms of incidence and mortality rates are a key element of the longitudinal design of the HCH cohort study, which will be available shortly.

Discussion: The response rate of this census follow-up, which serves as basis for upcoming longitudinal risk analyses and incidence estimates within the HCHS, was significantly higher compared to other cohort studies [3]. A concise design of the questionnaire, postal reminders, and the availability of telephone interviews are useful tools to considerably increase response among participants while adressing population groups that are usually hard to reach. Clinical end point adjudication is crucial to warrant reliable outcome data.

The authors declare that they have no competing interests.

The authors declare that an ethics committee vote is not required.