Abstract: A method for pre-analytical treatment of a serum or plasma sample from a patient suspected of suffering from oxidative stress, which includes contacting the sample with one or more microcapsules having a gelled alginate core and a semipermeable coating, where the alginate core includes dispersed receptors against an oxidised human parathyroid hormone (PTH) peptide. The semipermeable membrane can be obtained by layer-by-layer deposition of polycationic and polyanionic macromolecules onto the gelled core, following by hardening, crosslinking and co-acervation of the macroionic phases.