In addition, the L-kynurenine to L-tryptophan ratio (an index of indoleamine (2,3)-dioxygenase (IDO) activity) demonstrated a significant (P < 0.001) increase between the CRPS group and healthy controls, whereas the global arginine bioavailability ratio (GABR) (defined as L-arginine/[L-ornithine + L-citrulline]) was significantly (P < 0.0001) decreased in the CRPS subjects (Table 3). Here, IDO1 is linked to complex regional pain syndrome.