Additional unmet needs include: the ability to predict clinical response so that these drugs, which are expensive and have the potential for serious toxicity, can be targeted to patients who would most benefit [55]; an understanding of acquired drug resistance to anti-TNF agents [56]; a full explanation for why patients with spondyloarthritis (a group of disorders that includes AS and PsA) have a 20% lower probability of discontinuing TNF antagonists than patients with RA [57]; and an understanding of reasons for and predictors of discontinuation. Here, TNF is linked to rheumatoid arthritis.