Vivaspin 500 l concentrators

10 ng/mL LPS from E. coli was spiked into PBS (0.01 M), native serum, lipoprotein-depleted serum, lipoprotein-enriched serum and 4% (w/v) human serum albumin solution (Kedrion Biopharma, Barga, Italy). Each solution was filtered through a 0.22 µm and a 0.1 µm sterile filter from Merck (Burlington, USA), and centrifuged through Vivaspin 500 µL concentrators from Sartorius (Göttingen, Germany) with molecular cut offs of 100, 300 and 1000 kDa. In order to determine the sieving coefficient of LPS through the different filters, the LPS activity of the samples before and after filtration was determined using the LAL test.

Fresh serum from healthy donors was spiked with 3 µg/mL β2-microglobulin (Sigma-Aldrich, St.Louis, USA) and subsequently filtered through Vivaspin 500 µL concentrators from Sartorius (Göttingen, Germany) with molecular cut offs of 10, 30, 50, 100, 300 and 1000 kDa. In serum and the respective filtrate, the albumin and β2-MG concentrations were determined using Cobas c311 analyzer with the respective test kit. The sieving coefficient (S) in percent of albumin and β2-MG for each membrane was calculated using the following formula: $$S\left[\%\right]=\frac{C_{\mathit{serum}}}{C_{\mathit{filtrate}}}\times 100$$ where C_serum stands for serum and C_filtrate for filtrate concentration.
Additionally, the endotoxin recovery of 5 ng/mL (E. coli) in each filtrate was determined by LAL test.