This is partly owing to limited evidence for TDM of this drug, in contrast to the strong correlation described between tumor necrosis factor inhibitor serum levels, ADAs, and treatment response across multiple immune-mediated inflammatory diseases.16,17,18,19,20 Reports on the association between ustekinumab level and response to treatment have been inconclusive,21,22,23,24,25 with basic parameter requirements for TDM (eg, therapeutic range and target drug level) yet to be established in the context of psoriasis. This evidence concerns the gene AGPS and psoriasis.