Sample processing
RECRUITMENT
In IP12Flowchart showing IP12 recruitment and consent to provide blood samples.After reception at MRC Epidemiology Unit, venous blood samples for glycated haemoglobin were refrigerated until analysis. The remaining venous blood samples were centrifuged, and aliquots of plasma and serum were stored at − 70 °C until analysis. DBS samples were received at the University of Essex, stored at room temperature, batched and sent to MRC Epidemiology Unit where they were later frozen at − 70 °C until analysis. DBS samples were stored in small plastic bags with a sachet of desiccant. The DBS samples were graded by visual inspection and given two scores: (1) number of useable spots (0–5 useable spots or missing if no spots) and (2) if applicable, a quality code to describe the spots (operationalised as either: good quality with no issues, not dried, other issue, or not dried and additional issue). The latter is based on commonly used DBS grading charts