The 18 items' Non-Communicating Adult Pain Checklist (NCAPC) has been developed from the 27 items Non-Communicating Children Pain Checklist to better capture pain behavior of adults with Intellectual and Developmental Disabilities (IDD). As part of the NCAPC's measurement properties, internal consistency, reliability and sensitivity to pain have been evaluated and found satisfactory, using scores based on video-uptakes. The aim of the article therefore was to examine the instrument's discriminative ability and sensitivity to pain of adults at different levels of IDD when scored within a clinical situation as well as through video-uptakes. Participants were 59 adults at different levels of IDD who were observed for pain behavior, before and during dental hygiene treatment (scored directly) and influenza injection (scored from video-uptakes), using the NCAPC. The results suggest that the NCAPC differentiated between pain and non-pain situations, as well as between pain reaction during two different medical procedures expected to cause more or less pain, and it was found sensitive to pain at all levels of IDD. We conclude that the present findings add to previous findings of measurement properties of the NCAPC, and support that it can be scored directly in a clinical setting. (Contains 2 figures and 2 tables.)