A diagnostic method of monitoring anti-inflammatory and/or antioxidant actions of therapeutic agents comprises determining the level of at least one systemic marker indicative of inflammation or oxidation in a bodily sample taken from a subject at base line or following administration of the therapeutic agent. The marker includes at least one of MPO activity, MPO mass, select MPO-generated oxidation products, and combinations thereof. The level of the systemic marker is compared with a predetermined value to monitor the anti-inflammatory and/or antioxidant actions of the therapeutic agent.