Background: Healthcare professionals without specialist training in communication disorders may not know how to identify and support patients with communication disabilities during mental capacity assessments. To meet this need, a novel communication screening tool was developed and tested as part of a mental capacity assessment support toolkit. Aims: To provide an initial evaluation of the communication screening tool's usability, interrater reliability and criterion validity. Methods & Procedures: A prototype communication screening tool was developed iteratively using co-production and user-centred design principles. A mixed-methods case series design was used to explore how multidisciplinary healthcare professionals used the tool to test patients in acute hospital and intermediate care settings. Usability data were collected in an electronic survey and from a documentary analysis. Screening test outcomes obtained by pairs of professionals were compared to measure the tool's interrater reliability. Outcomes obtained by professionals were compared with the outcomes of a speech and language therapist's communication assessment to measure criterion validity. Quantitative data were analysed using frequency counts and inferential statistics. Qualitative data were analysed using framework analysis. Outcomes & Results: A total of 21 professionals, including physicians, nurses, occupational therapists, physiotherapists, and speech and language therapists, and 17 patients with diagnoses of stroke or cognitive impairment took part. Professionals reported they found the tool easy to use, useful and that its use increased their understanding of communication support methods and the speech and language therapist role in relation to mental capacity assessment. However, not all used it consistently or accurately. Professionals reported they chose not to use the tool when they perceived patients' communication to be intact. Four of eight patients with a diagnosis of dementia or memory impairment, who professionals elected not to screen, were found to have significant communication needs. Screening outcome data for nine patients suggest the tool's interrater reliability is currently moderate, whilst its criterion validity is poor. Conclusions & Implications: This study highlights that non-speech and language therapist health professionals have difficulty identifying and screening for communication difficulties. This confirms existing evidence that people with communication disabilities may not receive the decision-making support they require during mental capacity assessments when speech and language therapists are not involved. Greater understanding of health professionals' thought processes regarding communication is required to further develop this unique communication screening tool so that it can effectively enable healthcare professionals to identify and use communicative adaptations to support decision-making.