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ABSTRACT. Objective. To summarize the work performed by the Outcome Measures in Rheumatology (OMERACT) Ultrasound (US) Working Group on the validation of US as a potential outcome measure in gout. Methods. Based on the lack of definitions, highlighted in a recent literature review on US as an outcome tool in gout, a series of iterative exercises were carried out to obtain consensus-based definitions on US elementary components in gout using a Delphi exercise and subsequently testing these definitions in static images and in patients with proven gout. Cohen's κ was used to test agreement, and values of 0-0.20 were considered poor, 0.20-0.40 fair, 0.40-0.60 moderate, 0.60-0.80 good, and 0.80-1 excellent. Results. With an agreement of > 80%, consensus-based definitions were obtained for the 4 elementary lesions highlighted in the literature review: tophi, aggregates, erosions, and double contour (DC). In static images interobserver reliability ranged from moderate to almost perfect, and similar results were found for the intrareader reliability. In patients the intraobserver agreement was good for all lesions except DC (moderate). The Outcome Measures in Rheumatology (OMERACT) US Working Group (Appendix 1) developed a gout subgroup with the purpose of validating US as an imaging tool for gout. If this objective is achieved, US may be implemented as an outcome measure in gout. Is Ultrasound a Validated Outcome Measure in Gout? In 2013, a systematic literature review was published evaluating US as an outcome tool in gout and asymptomatic hyperuricemia 10 . The report found 18 out of 67 articles published since 1975 to be eligible for review. Described in the literature were 4 main pathologies related solely to gout: tophi, double contour sign (DC), soft tissue abnormalities, and bony lesions. The review highlighted that US was able to detect tophi using magnetic resonance imaging (MRI) as a gold standard, and this measure was found sensitive to change. The DC is an articular cartilage abnormality related to the deposition of crystals on the surface of the hyaline cartilage, which seemed specific to gout, with excellent inter-reader reliability and sensitive to change (the latter only in a very small patient population). Soft tissue pathology such as intrasynovial hyperechogenicity may be indicative of gout. US was less sensitive than MRI for diagnosing erosions (bony lesions) but more sensitive than conventional radiography, as is also known from rheumatoid arthritis studies Criterion and construct validity were assessed only for tophi, and overall there was a lack of consensus on the definitions of the 4 elementary lesions and their validity according to the OMERACT filter 13 . Current Limitations of US in Gout Assessment Despite clear interest in this imaging technique for the management of gout, the literature review clearly pointed to a lack of clear US definitions for the main 4 elementary lesions identified: tophi, DC, soft tissue hyperechogenicity (punctuate crystal aggregates), and bony lesions (erosions). This lack of consensus-based definitions impairs the ability to validate US according to the OMERACT filter and hampers widespread use of US in therapeutic clinical trials, due to the difficulty to measure the same phenomenon. In order to implement US in the management of patients with established or suspected gout the "gout subgroup of the OMERACT US Working Group" initiated a validation process. The first step was to obtain consensus-based definitions for the US elementary lesion as indicated by the literature review. This was accomplished by performing a Delphi exercise 14 . Thirty-five rheumatologists performing US and with an interest in gout were invited to participate, and 32 responded positively. After 3 Delphi rounds, > 80% agreement was obtained for each definition Agreement was obtained to use the existing definitions for both synovitis and tenosynovitis 15 because these may be co-components in gout disease. Agreement could not be obtained to include synovitis (including Doppler activity) as an elementary lesion indicative of gout, because the presence of synovitis alone was not considered specific enough to define gout disease because it is a key component in other inflammatory arthropathies as well 14 . On the other hand, even if erosions may also be seen in other arthropathy conditions, since they may also be found extraarticularly in gout and may possibly have a slightly different appearance, it was decided to test, as part of the Delphi exercise, whether the existing definition worked also in gout. Perfect agreement was obtained to keep the definition close to the definition used for erosions in general. The second step was to test the reliability of the obtained definitions in a Web exercise consisting of static images of the elementary lesions. The Web exercise included 110 US images of the 4 lesions obtained from feet and knees and 20 of these images were shown twice in order to test both interand intrareader reliability. Twenty-seven of the 35 rheumatologists participating in the Delphi exercise participated in the reliability study. Cohen's κ was used to evaluate interand intrareader reliability. Κ values 0-0.20 were considered poor; 0.20-0.40 fair; 0.40-0.60 moderate; 0.60-0.80 good; and 0.80-1 excellent The third step was to test the agreement and reliability of the elementary lesions in a cohort of patients with gout. Sixteen of the rheumatologists previously involved in the first and second step participated in a workshop with 8 patients with crystal-proven gout. Both intra-and inter-reader reliability was assessed by scanning the patients twice within the same day. The areas of attention were the intercondylar region of the knee, the 1st metatarsophalangeal (MTP) joint, and the patellar tendon. Cohen's κ was used to evaluate inter-and intrareader reliability. Κ values of 0-0.20 were considered poor; 0.20-0.40 fair; 0.40-0.60 moderate; 0.60-0.80 good; and 0.80-1 excellen