Method of preparing glass fiber filter

A method for preparing a plasma or serum sample from a whole blood sample comprising contacting a glass fiber filter containing calcium, sodium and potassium components with an aqueous solution of a compound capable of eluting calcium, sodium and potassium components from the glass fiber filter, said compound being selected from the group consisting of 0.1 μM to 1M of an organic acid and 0.1 μM to 10 mM of a mineral acid, the contacting being carried out for from 1 second to 60 minutes to elute the calcium, sodium and potassium components in the glass fiber filter into the aqueous solution of the compound, separating the aqueous solution from the glass fiber filter and washing the glass fiber filter with water to remove any residual aqueous solution adhered to the glass fiber filter and then filtering the whole blood sample through the thus treated glass fiber filter.