Participants were met at their homes or on campus by a research assistant and a phlebotomist. First, participants completed a consent form and a questionnaire on demographics (e.g., age) and, if female, on their pregnancy history. A 10-mL sample of whole blood was drawn from the participants using sodium heparin coated vacutainers (BD Biosciences). Participants were then debriefed and compensated financially.
The whole-blood sample was processed in our laboratory after extraction, typically within 3–4 h. For the purpose of this study, we extracted the plasma using the Ficoll method. Whole blood was mixed with an equal volume of sterile PBS solution and overlaid drop-by-drop on 10 mL of Ficoll in a 50-mL tube. The tube and its contents were centrifuged without braking for 30 min at 600 × g at room temperature. After centrifugation, the upper plasma layer was carefully extracted into 2-mL aliquots and stored at −80 °C or in liquid nitrogen until ELISAs were conducted.
The whole-blood sample was processed in our laboratory after extraction, typically within 3–4 h. For the purpose of this study, we extracted the plasma using the Ficoll method. Whole blood was mixed with an equal volume of sterile PBS solution and overlaid drop-by-drop on 10 mL of Ficoll in a 50-mL tube. The tube and its contents were centrifuged without braking for 30 min at 600 × g at room temperature. After centrifugation, the upper plasma layer was carefully extracted into 2-mL aliquots and stored at −80 °C or in liquid nitrogen until ELISAs were conducted.