Current methods measure glutathione levels only, which require cellular extraction, rather than the glutathione recycling dependent antioxidant activity in intact cells. Here, we present a novel method using a bioactive probe of the oxidative pentose phosphate cycle, termed the OxPhos™ test, to quantify glutathione recycling dependent antioxidant activity in whole blood and intact human and rodent cells without the need for the isolation and cytoplasm extraction of cells.