At baseline a 10 mL non-fasting blood sample was collected from each participant and the time since last meal was recorded. The samples were maintained at 0-4°C and shipped to the local study laboratory for aliquotting into three plasma cryovials and one DNA-containing buffy coat cryovial. They were then stored in a -40°C freezer for three to four months before being couriered on dry ice to the central blood repository in Beijing for storage at -80°C or liquid nitrogen tanks. Two of the frozen plasma aliquots from each participant were couriered to Oxford and stored in liquid nitrogen tanks.
Repeat blood samples and other biological samples (e.g. urine, saliva and faecal) from participants, were collected during subsequent resurveys.
Various blood tests were conducted at each survey, either on-site using blood spot tests e.g. blood glucose, hepatitis B surface antigen (HBsAg) or at local laboratory using desktop analysers e.g. lipid-related biomarkers.