In clinical diagnostics, the fluidic part for sample-preparation should not absorb any of the substances of interest. In this study, the absorption of 4 different interleukins was investigated in a newly developed microfluidic blood filtration element (BFE). The BFE is capable of extracting 8-15 μL volume of plasma from 100 μL of whole blood in 7 minutes. Whole blood from 4 separate human subjects was tested. The recovery from our BFE was lower than 100%, but, as it was reproducible (except for TNF-α), enables use of the device in clinical trials with prior calibration.