Background The self-administered comorbidity questionnaire (SCQ) was developed to understand the impact of 13 common comorbidities on functioning and resource utilisation. In patients with AS, we recently showed that criterion- and construct validity of the SCQ was good, and improved after removing rheumatic items from the questionnaire . However, this modification (SpA-SCQ) does not address uveitis, psoriasis and inflammatory bowel disease, which are frequently occurring extra-articular manifestations (EAMs) in SpA that might have a relevant impact on disease specific and generic outcomes.
Objectives To assess and compare criterion and construct validity of the SpA-SCQ and the SpA-SCQ with items on EAMs (“SpA-SCQ-EAM”).
Methods Data from the CO-MOrbidities in SPA (COMOSPA) study, conducted in 22 countries, were used. Criterion validity was defined as the agreement between the EAMs reported by patients and diagnosed by the physician. Improvement in construct validity was first assessed by calculating Spearman coefficients for the SpA-SCQ and SpA-SCQ-EAM with demographics, physical function (BASFI or a simple average of the short form of the HAQ (mHAQ)), generic health utility (EQ-5D) and disease activity (ASDAS-CRP). Next, an age-gender adjusted linear regression analysis with health outcomes was performed to quantify the contribution of these EAM to the SpA-SCQ. Finally, we compared the variance explained by the linear regression analysis (adjusted for age and gender) on the association of the SpA-SCQ, SpA-SCQ-EAM and the rheumatic disease comorbidity index (RDCI) with health outcomes.
Results 3,984 patients (mean age 43.6 (SD 13.9) years, 2588 (65.0%) male, mean disease duration 8.2 (SD 9.3) years, of which 2,217/3,067 (72.4%) HLA-B27 positive) contributed to the analysis. The agreement between patient and physician diagnosed EAMs was substantial to almost perfect (uveitis κ=0.81, IBD κ=0.74, psoriasis κ=0.78), and was not systematically different across countries. Corresponding weak to moderate correlations were seen for either the SpA-SCQ or the SpA-SCQ-EAM with age, BASFI and mHAQ (0.27 to 0.43), and EQ-5D (-0.33 to -0.34). Poor correlations were found for both SpA-SCQ/SpA-SCQ-EAM and ASDAS-CRP (0.10 and 0.08, respectively). Table 1 shows that adding the EAMs to the SpA-SCQ did not result in relevant improvement of the age/gender adjusted influence of the SpA-SCQ on the BASFI. Similar results were found with respect to the other health outcomes. Finally, both the SpA-SCQ and the SpA-SCQ-EAM performed somewhat better with respect to the explained variance in health outcomes than the RDCI (Table 1).
Conclusions EAMs can be reliably assessed through self-report by patients with SpA. Adding EAM to the SpA-SCQ does not contribute to explaining disease-specific or generic outcomes, and is therefore not necessary. Both the SpA-SCQ and SpA-SCQ-EAM are superior to the RDCI when adjusting health outcomes for co-existing diseases in patients with SpA.
Stolwijk C et al, Rheumatology 2014;53:1054-1064.
Acknowledgements Study was conducted under the umbrella of ASAS and supported by unrestricted grants from Abbvie, Pfizer and UCB.
Disclosure of Interest None declared