PDXP and rheumatoid arthritis: For example, in a study of patients with rheumatoid arthritis, inflammation was associated with low plasma PLP concentrations but normal erythrocyte PLP concentrations and urinary PA excretion, and in the same study the authors showed that, in an animal model of arthritis, inflammation was associated with lower circulating and liver PLP concentrations but normal PA excretion and PLP storage in muscle (Chiang et al., 2005).