Specimen Collection and Preparation
Overnight 12 hours’ fasting is required before intravenous blood sampling in the morning. The blood withdrawal volume is approximate 1 mL for generating 400 μL serum. The same brand of glass centrifuge tubes (BD Vacutainer) was used for blood collection. To avoid the metabolite changes before preprocessing, blood samples were temporarily stored at 4 ℃ until natural coagulation occurred. Serum was prepared by 2,000g centrifugation for 10 minutes at 4 ℃ after blood clotting. All serum samples were saved under -80 ℃ for long–term storage until use.