KP metabolite features that effectively predicted PD diagnosis included the KMO-related product/substrate ratio 3-HK/Kyn, 3-HK/KA, 3-HKr, 3-HK, PLP, QA/KA, KA, and TRP in plasma and QA/KA, 3-HK/KA – an excitotoxic ratio36 – along with HKr, KA, PL, PLP, 3-HK and 3-HAA in CSF (Fig. 4c). Here, KMO is linked to Parkinson disease.