Figure 4

The serum samples were obtained after the blood was collected and maintained at 4℃ for 30 min. As for the freeze-drying method, serum samples were placed into the inner tube and ultrasonically blended for 5 min. The filtered sample flowed out from the inner tube and was reserved in the outer tube. The inner tube was then freeze-dried in a vacuum freeze dryer. After completing all programmed settings, the freeze-dried inner tubes were removed and eventually stored at 4℃ until analysis. Regarding the conventional method, the serum samples were transferred to polyethylene tubes and then stored in an ultra-low temperature freezer at – 80 °C until the time of the assay.