How was the baseline survey conducted?
For the baseline survey, a Regional Coordinating Centre (RCC) and survey team, consisting of about 15 full-time staff with relevant health-related qualifications and fieldwork experience, were established in each of the 10 study areas. Potentially eligible participants in each of 100-150 administrative units (rural villages or urban residential committees) selected for the study within each region were identified through official residential records, and invitation letters (with study information leaflets) were delivered door-to-door by local community leaders or health workers following extensive publicity campaigns. As a prerequisite for participating, all participants were asked to bring their unique national ID cards to the assessment centre set up in the local community. After registration and giving informed written consent (which allows access to their medical records and long-term storage of blood for anonymised and non‑specified medical research purposes), each participant moved through various stations in the assessment centre. The whole clinic visit typically took about 60-75 minutes to complete. The target daily recruitment rate was 70-80 participants per region.
How were the blood samples collected and processed?
For each participant, a 10 ml non-fasting blood sample was collected. The samples were shipped, while being maintained at 0-4°C, to the local study laboratory for aliquotting into three plasma cryovials and one DNA-containing buffy coat cryovial. They were all stored in a -40°C freezer for 3-4 months then couriered on dry ice to the central blood repository in Beijing for storage at -80°C. Twice a year, two of the frozen plasma aliquots from each participant were couriered to Oxford and stored in liquid nitrogen tanks.
What quality control measures were implemented?
All study materials and equipments were supplied centrally. Extensive training was provided to the local survey team. The field survey in each region usually started immediately after completion of training, under the direct supervision and support of the research team from Oxford and Beijing for about one week to ensure that all procedures were followed properly.
Within a few weeks of the initial baseline survey in a particular community (e.g. village), a quality control survey was done, involving about 3% of participants randomly selected from that community with repeat questionnaire and measures of selected items. Regular central monitoring was also undertaken to assess the recruitment rate, the distributions of certain key variables, delay time with blood processing, and the consistency of the data collected both overall and by individual staff. On-site monitoring visits were also undertaken every 6 months by provincial CDC staff and annually by Oxford/Beijing staff.