On the measurement of circulating antioxidant capacity and the nightmare of uric acid

Summary 1. In recent years, evolutionary ecologists have become increasingly interested in antioxidants and oxidative stress. Information on redox systems can provide new insights into our understanding of life‐history variation and animal responses to environmental stressors. 2. A common approach of ecological studies to the study of antioxidant capacity of animals has been measurement of the total antioxidant capacity of serum or plasma. Some of these studies have suggested that most of the antioxidant capacity measured in plasma is made up of uric acid and, therefore, estimates of antioxidant capacity should be corrected for the concentration of uric acid. 3. Here, I show that (i) the correlation between plasma concentration of uric acid and plasma antioxidant capacity is method dependent and (ii) different assays for the quantification of circulating antioxidant capacity can provide information on different components of the antioxidant machinery. 4. To determine whether measurements of antioxidant capacity need to be corrected for the uric acid concentration in the sample, it is therefore important to take into account the biochemical properties of the assay used.