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•Updating strategies can be optimized with prioritization processes that help identify CGs, CG sections, CQs, or recommendations in the greatest need for updating.•We recently developed the UpPriority tool, a pragmatic tool for prioritizing CG questions for updating. The tool was based on a published methodological systematic review and a multistep process involving relevant stakeholders.•We applied the UpPriority tool to a set of CGs using a step-by-step process that included: 1) establishment of the UpPriority Implementation Working Group, 2) mapping of the CG questions and recommendations, 3) development of a survey to prioritize CQs, 4) assessment of CQ's priority according to six items, 5) calculation and ranking of priority scores, 6) decision of prioritized CQs for updating, and 7) development of the priority report. We assessed the tool implementation process (appraisers’ experience when using the tool) and the inter-observer reliability of the tool, and we provided suggestions for improvement.•The UpPriority is a useful tool to identify which CQs within a CG need to be prioritized for update in a real-world scenario. We aim to 1) use the UpPriority tool to identify which clinical questions (CQs) within the clinical guidelines (CGs) need to be prioritized for updating and 2) assess the implementation of the tool in a real-world set of CGs. We systematically assessed CQs from a sample of CGs developed in the Spanish National Health System CG program. We applied the UpPriority tool to each CG using a step-by-step process that included: 1) establishment of the UpPriority Implementation Working Group, 2) mapping of the original CG questions and recommendations, 3) development of a survey to prioritize CQs, 4) assessment of CQ's priority according to six items, 5) calculation and ranking of priority scores, 6) decision of prioritized CQs for updating, and 7) development of the priority report. We assessed the tool implementation process (appraisers’ experience when using the tool) and the inter-observer reliability of the tool, and we provided suggestions for improvement. We included four CGs with a total of 107 CQs on the following topics: chronic heart failure (10 CQs), inherited retinal dystrophies (39 CQs), menopause (20 CQs), and open-angle glaucoma (38 CQs). We included a total of 30 participants, most of them clinicians that were members of the original CG development groups. CQs were classified in three groups: 1) high priority (CQs prioritized for updating [16/107; 15.0%]), 2) medium priority (CQs that could be prioritized for updating [47/107; 43.9%]), and low priority (CQs that were not prioritized for updating [44/107; 41.1%]). The mean time each appraiser needed to assess the CQs with the tool was 3.8 hours (range 0.5 to 10). Agreement among the appraisers varied among the CGs. Appraisers considered that the tool was useful. We suggest some areas for consideration when using the tool including: 1) identification of key appraisers, 2) customization of training materials, 3) establishment of priority thresholds, and 4) provision of methodological support. The UpPriority is a useful tool to identify which CQs within a CG need to be prioritized for update in a real-world scenario. Recruitment and training of topic experts are the main challenges when using the tool.