In addition to the assessment of the efficacy of solid malignant tumor therapy, the determination of IDO activity can also be applied to the diagnosis and therapeutic monitoring of various diseases, such as kidney disease, intestinal disease, cardiovascular disease, reproductive system, parasitic infections, mental illness, etc. At present, the determination of IDO activity in human plasma/serum has great application prospects in the diagnosis and treatment of a variety of tumor-related diseases. Here, IDO1 is linked to psychiatric disorder.