A simple pretreatment is to add 10% TCA (trichloroacetic acid) aqueous solution in the ratio of 1:1. Centrifuge and collect the supernatant.
Another simple way to pretreat whole blood is to perform protein precipitation with cold acetonitrile-methanol (85:15) in the ratio of 1:2. After centrifuging, the supernatant is diluted with solvent appropriate for extraction loading.
A more thorough pretreatment is converting whole blood into blood plasma. Centrifuging anti-coagulated blood gives blood plasma as supernatant. Blood plasma preparation kits with prepackaged tubes containing an anticoagulant are commercially available.