A method for obtaining blood plasma from a whole blood sample comprising the following steps a) contacting the whole blood sample with a composition (A) comprising at least one carboxylic acid, wherein the addition of the acidic composition (A) and optionally further additives to the whole blood sample provides a sample mixture having a pH that lies in a range from 2.5 to 5; b) binding red and white blood cells to a magnetic solid phase; wherein step a) and step b) can be performed sequentially or simultaneously, c) separating the solid phase with the bound cells from the remaining sample thereby providing blood plasma.