Phenobarbital derivatives synthesized out of the alkyl chain at the 5-position, particularly with hydrophilic properties, and carrying an active ester at the end, allow formation of aminodextran conjugates that give curves in the desired range of the assay in the ONLINE TDM microparticle assay format when matched against the Roche FPIA antibody specific for phenobarbital (“an antibody specific for phenobarbital”).