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Context.--Laboratories must demonstrate analytic validity before any test can be used clinically, but studies have shown inconsistent practices in immunohistochemical assay validation. Objective.--To assess changes in immunohistochemistry analytic validation practices after publication of an evidence-based laboratory practice guideline. Design.--A survey on current immunohistochemistry assay validation practices and on the awareness and adoption of a recently published guideline was sent to subscribers enrolled in one of 3 relevant College of American Pathologists proficiency testing programs and to additional nonsubscribing laboratories that perform immunohistochemical testing. The results were compared with an earlier survey of validation practices. Results.--Analysis was based on responses from 1085 laboratories that perform immunohistochemical staining. Of 1057 responses, 65.4% (691) were aware of the guideline recommendations before this survey was sent and 79.9% (550 of 688) of those have already adopted some or all of the recommendations. Compared with the 2010 survey, a significant number of laboratories now have written validation procedures for both predictive and nonpredictive marker assays and specifications for the minimum numbers of cases needed for validation. There was also significant improvement in compliance with validation requirements, with 99% (100 of 102) having validated their most recently introduced predictive marker assay, compared with 74.9% (326 of 435) in 2010. The difficulty in finding validation cases for rare antigens and resource limitations were cited as the biggest challenges in implementing the guideline. Conclusions.--Dissemination of the 2014 evidencebased guideline validation practices had a positive impact on laboratory performance; some or all of the recommendations have been adopted by nearly 80% of respondents. (Arch Pathol Lab Med. 2017;141:1247-1254; doi: 10.5858/arpa.2016-0558-CP), The College of American Pathologists (CAP) Pathology and Laboratory Quality Center (the CAP Center) develops and maintains evidence-based laboratory practice guidelines (LPGs) that follow the US Institute of Medicine's Clinical [...]