OBJECTIVES: To investigate the impact of obesity on response to the first TNF-α inhibitor (TNFI) treatment course in patients with PsA followed in routine care.

METHODS: We performed an observational cohort study based on the Danish and Icelandic biologics registries. Kaplan-Meier plots, Cox and logistic regression analyses were performed to study the impact of obesity (BMI ⩾30 kg/m(2)) on TNFI adherence and response after 6 months (according to 20/50/70% improvement in ACR criteria and EULAR criteria). Subanalyses studied the impact of obesity according to gender, TNFI type and nationality.

RESULTS: Among 1943 PsA patients (193 Icelandic/1750 Danish) identified in the registries, 1271 (65%) had available BMI and 408 (32%) were obese. The median follow-up-time was 1.5 years [interquartile range (IQR) 0.5-3.9]. Obese patients had higher baseline disease activity, for example, 28-joint DAS [mean 4.6 (sd 1.2) vs 4.4 (1.2)]; CRP [median 9 mg/l (IQR 5-19) vs 7 (3-18)] and visual analogue scale-pain [66 mm (IQR 48-76) vs 60 (38-74)], compared with non-obese patients (all P < 0.05). TNFI adherence was shorter in obese patients, especially among men, where the median TNFI duration was 2.5 years (95% CI 1.7, 3.2) in obese vs 5.9 (4.1, 7.7) in non-obese patients (P < 0.01). A EULAR good or moderate (EGOM) response was achieved by 55% of obese vs 65% of non-obese patients after 6 months (P = 0.02). In multivariable analyses, obesity increased the risk of TNFI withdrawal [hazard ratio 1.6 (95% CI 1.3, 2.0)] and reduced odds for EGOM response [odds ratio 0.47 (95% CI 0.29, 0.72)]. The impact of obesity was significant across genders, TNFI types and nationality.

CONCLUSION: Obesity was associated with higher disease activity and seemed to diminish response and adherence to TNFIs in PsA.