Undifferentiated arthritis: a changing population who did not benefit from enhanced disease-modifying anti-rheumatic drug strategies-results from a 25 year longitudinal inception cohort.

OBJECTIVES: International guidelines stress timely DMARD initiation in early arthritis as well as when classification criteria are not yet fulfilled. Consequently, undifferentiated arthritis (UA) patients may be increasingly treated with DMARDs. Since UA is a diagnosis of exclusion, the introduction of the 2010 classification criteria presumably decreased the UA population, as former UA patients became regarded as RA. Consequently, the contemporary definition of UA has changed into: no clinical diagnosis and not fulfilling the 1987 nor 2010 RA-criteria. Importantly, placebo-controlled trials on DMARD efficacy in contemporary UA are absent. We aimed to study whether enhanced treatment strategies across the last 25 years improved outcomes in contemporary UA, whereby inclusion period was used as instrumental variable for DMARD treatment.

METHODS: UA was defined, retrospectively, as clinical arthritis (joint swelling at physical examination) neither fulfilling the 1987 nor 2010 RA-criteria or any other clinical diagnosis. In total, 1132 UA patients consecutively included in the Leiden Early Arthritis Clinic between 1993 and 2019 were divided into five inclusion periods: 1993-1997, 1998-2005, 2006-2010, 2011-2014 and 2015-2019. The frequency of DMARD initiation was compared across the inclusion periods, as were the following outcomes: 28-joint DAS with CRP (DAS28-CRP) and the HAQ Disability Index (HAQ-DI) during follow-up, prevalence of DMARD-free-status within 10 years (DFS; spontaneous remission or sustained remission after DMARD stop) and progression to RA (according 1987/2010 criteria).

RESULTS: The contemporary UA population is mainly autoantibody negative, with a median swollen joint count of 2, tender joint count of 3 and HAQ score of 0.6. These characteristics were similar across the inclusion periods. DMARD treatment increased from 17% (1993-1997) to 52% (2015-2019) and methotrexate became more common. The DAS28-CRP during follow-up improved from 2011 onwards (-0.18 to -0.25 DAS units; P < 0.05). Disability scores during follow-up did not significantly improve. DFS prevalence also remained similar: 58%, 57% and 61% for 1993-1997, 1998-2005 and 2006-2010, respectively (P = 0.77). Likewise, the percentages of RA development did not decrease (14%, 21%, 26%, 18% and 27%, respectively).

CONCLUSION: Although intensified DMARD treatment slightly improved disease activity scores, physical functioning and long-term outcomes did not improve. This suggests overtreatment in the contemporary UA population and underlines the importance of developing stratification methods suitable for this patient-population.