KMO and diabetes mellitus: Overall, while the KYNA/KYN ratio (for estimating KAT activity) and the 3-HK/KYNA ratio (for assessing the balance between the neurotoxic and neuroprotective branches of the pathway) provide valuable indirect measures, our findings indicate that alterations are mainly observed in KMO activity in specific subgroups (i.e., diabetes and glaucoma) and in KAT activity across SPONCS severity levels.