Assessing the sensitivity and specificity of myositis-specific and associated autoantibodies: a sub-study from the MyoCite cohort.

OBJECTIVES: Myositis-specific and associated autoantibodies are important biomarkers in routine clinical use. We assessed local testing performance for myositis autoantibodies by comparing line immunoassay (LIA) to protein radio-immunoprecipitation and identifying clinical characteristics associated with each myositis autoantibody in the MyoCite cohort. METHODS: Serum samples from patients within the MyoCite cohort, a well-characterized retro-prospective dataset of adult and juvenile idiopathic inflammatory myopathy (IIM) patients in Lucknow, India (2017-2020), underwent LIA at Sanjay Gandhi Postgraduate Institute of Medical Science (SGPGIMS), Lucknow. Immunoprecipitation of 147 IIM patients' serum samples (125 adult-onset, 22 juvenile-onset) was conducted at the University of Bath, with researchers blind to LIA results. LIA performance was assessed against immunoprecipitation as the reference standard, measuring sensitivity, specificity and inter-rater agreement. Univariate and multivariate logistic regression determined clinical associations for specific myositis-specific autoantibodies. RESULTS: Immunoprecipitation identified myositis autoantibodies in 56.5% (n = 83) of patient samples, with anti-Jo1 (n = 16; 10.9%) as the most common, followed by anti-MDA5 (n = 14, 9.5%). While LIA showed good agreement for anti-Jo1, anti-PL7 and anti-PL12 (Cohen's κ 0.79, 0.83 and 1, respectively), poor agreement was observed in other subgroups, notably anti-TIF1γ (Cohen's κ 0.21). Strongly positive samples, especially in myositis-specific autoantibodies, correlated more with immunoprecipitation results. Overall, 59 (40.1%) samples exhibited non-congruence on LIA and immunoprecipitation, and κ values for LIAs for anti-TIF1γ, anti-Ku, anti-PmScl, anti-Mi2 and anti-SAE ranged between 0.21 and 0.60. CONCLUSION: While LIA reliably detected anti-Jo1, anti-PL7, anti-PL12, anti-MDA5 and anti-NXP-2, it also displayed false positives and negatives. Its effectiveness in detecting other autoantibodies, such as anti-TIF1γ, was poor.

Incidence of Psoriatic Arthritis in a Primary Care Psoriasis Population in the United Kingdom.

OBJECTIVE: To determine the annual incidence of psoriatic arthritis (PsA) in a United Kingdom primary care population with preexisting psoriasis (PsO) followed prospectively over 2 years after excluding baseline prevalence of existing disease. METHODS: Total Burden of Psoriasis (TUDOR; ISRCTN registry: ISRCTN38877516) was a multicenter, prospective, 2-arm parallel-group cluster randomized controlled trial of the early identification of PsA by annual rheumatological assessment (termed "Enhanced Surveillance") vs standard care in people with PsO identified in primary care. Incidence of PsA is reported at 12 months and 24 months using patients from the Enhanced Surveillance arm, which allows for the exclusion of patients with prevalent PsA at baseline. RESULTS: Fourteen of 511 participants attending a 12-month screen developed PsA over that interval, giving an incidence of 2.74/100 patient-years (PYs; 95% CI 1.32-4.16). Another 7/444 participants attending the 24-month visit developed PsA, giving an incidence of 1.58/100 PYs (95% CI 0.42-2.74). The combined incidence over 2 years was 2.20/100 PYs (95% CI 1.27-3.13). CONCLUSION: The estimated annual incidence of PsA over a 2-year period was 2.20/100 PYs, which is in keeping with studies including clinical assessment rather than relying on health records alone. Extended follow-up of the TUDOR cohort with accrual of larger numbers of incident cases will allow risk factors for PsA to be explored in more depth.