Background In clinical practice, a dichotomous approach to delirium identification may no longer be relevant when existing delirium screening tools measure a range of scores. The objective of this study was to compare the Confusion Assessment Method for the Intensive Care Unit 7-item (CAM-ICU-7) and the Intensive Care Delirium Screening Checklist (ICDSC) as measures of the spectrum of delirium severity in critically ill adult patients. Methods In this cross-sectional study, 218 patients underwent 641 paired assessments by bedside nurses (ICDSC, as per usual care) and trained research assistants (CAM-ICU-7). Correlation between the CAM-ICU-7 and ICDSC scores was evaluated. Logistic regression was used to explore associations between CAM-ICU-7 or ICDSC score and length of ICU stay and mechanical ventilation (receipt, ≥96 hours). Results Delirium prevalence evaluated by the CAM-ICU-7 and ICDSC were 46.3% (95% CI:39.7–53.0) and 34.4% (95% CI:28.3–41.0). Prevalence of less than clinical threshold symptoms of delirium evaluated by the CAM-ICU-7 (score: 1–2) and ICDSC (score: 1–3) were 30.3% (95%CI:24.5–36.7) and 50.9% (95%CI:44.3–57.6). The CAM-ICU-7 and ICDSC had significant positive correlation (0.58, p<0.001). Agreement between the tools as measures of delirium was moderate (kappa = 0.51) and as measures of less than clinical threshold symptoms of delirium was fair (kappa = 0.21). Less than clinical threshold symptoms of delirium identified by the ICDSC, not CAM-ICU-7, were associated with prolonged length of ICU stay (≥7 days) in patients <65 years of age [Odds Ratio (OR) 9.2, 95% CI:2.5–34.0] and mechanical ventilation (receipt: OR 2.8, 95% CI:1.3–6.4; ≥96 hours: OR 6.6, 95% CI:1.9–22.9), when compared to patients with no delirium. Conclusions The CAM-ICU-7 and ICDSC are measures of the spectrum of delirium severity that are closely correlated. Less than clinical threshold symptoms of delirium measure by the ICDSC is a better predictor of outcomes, when compared with the CAM-ICU-7.