Objective: Nursing workload has been shown to have negative impacts on job satisfaction, retention and turnover. The NAS is one of existing tools targeting nursing workload quantification. Although tested in multiple settings, few studies explored nurses’ perception of its representativeness of workload, and its impacts when used to readjust nurse/patient ratios. This study was conducted to validate nurses’ perception of representativeness and potential usefulness of the Nursing Activities Score (NAS) in regard to nurses’ perceived workload and explore its impacts as part of a workload readjustment initiative.Methods: A mixed method design was selected, combining semi-structured interviews (n = 13), and secondary analysis of project monitoring data for an entire intensive care unit (n = 139). Statistical analysis was performed using SPSS v.22 software for quantitative data, and qualitative data was analyzed through NVivo 12 software using a thematic deductive-mixed content analysis, aiming for the emergence of recurring themes. Results: When taken as a whole, the NAS is perceived to be representative of nursing workload. However, validity concerns were identified at the tool completion level, notably in regard to improper documenting of events and fundamental understanding of the tool by nurses. Numerous impacts related to the use of the NAS were also identified.Conclusions: Although the NAS appears to adequately represent nursing workload, its pertinent utility remains debatable due to high subjectivity described in this study. The sole use of the tool for patient assignment is therefore questionable.