Serum tryptophan and KP metabolites, primarily comprising, KYN, KYNA, 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HAA), AA, picolinic acid and quinolinic acid, were hence measured in, and compared between, cognitively normal individuals with preclinical AD characterised by high NAL (NAL+; standard uptake value ratio (SUVR) ≥1.35) and individuals with no apparent risk to AD, characterised by low NAL (NAL−, SUVR <1.35)16–18. Here, NPPA is linked to Alzheimer disease.