In achievement testing we assume that students will demonstrate their maximum performance as they encounter test items. Sometimes, however, student performance can decline during a test event, which implies that the test score does not represent maximum performance. This study describes a method for identifying significant performance decline and investigated its utility as an indicator of generalized test-taking disengagement. Analysis of data from a computerized adaptive interim achievement test showed that performance decline classifications exhibited characteristics similar to those from disengagement classifications based on rapid guessing. More importantly, performance decline was found to identify disengagement by many students who would not have been identified as disengaged based on rapid-guessing behavior.