Healthcare Glossary→Patient Acuity
Clinical Measures

Patient Acuity

Patient acuity refers to the degree of illness severity and the intensity of care required by a patient — a clinical concept that underpins staffing models, payment systems, and quality benchmarking across all care settings.

What is Patient Acuity?

Patient acuity describes the clinical severity and complexity of a patient's condition and the resultant nursing, physician, and ancillary care intensity required. High-acuity patients have severe, unstable conditions requiring intensive monitoring and intervention; low-acuity patients have stable conditions requiring routine care. Acuity is measured differently across care settings — from validated ICU severity scores to DRG relative weights for inpatient payment to CMS Hierarchical Condition Categories for outpatient risk adjustment.

ICU Severity Scoring

  • APACHE II (Acute Physiology and Chronic Health Evaluation II): Widely used ICU severity score (0–71) calculated from 12 physiological variables plus age and chronic health points. Scores ≥25 are associated with > 50% predicted mortality in many patient populations. Used for ICU benchmarking, research cohort definition, and quality comparison.
  • SOFA (Sequential Organ Failure Assessment): Assesses six organ systems (respiratory, coagulation, liver, cardiovascular, CNS, renal) on a 0–4 scale each. Total SOFA score 0–24; scores ≥11 are associated with high mortality. Also used in the Sepsis-3 definition (qSOFA for rapid screening).
  • SAPS II (Simplified Acute Physiology Score): Alternative to APACHE II, using 17 variables. More commonly used in European ICUs.

DRG Relative Weight as Acuity Proxy

For inpatient payment and benchmarking, DRG relative weight serves as a practical acuity proxy. A DRG with a relative weight of 3.0 represents approximately 3× the average resource consumption of the base DRG (relative weight 1.0). Case Mix Index — the average relative weight across all discharges — provides a population-level acuity measure for hospital-to-hospital comparison. Acuity-adjusted LOS benchmarks (expected LOS given the DRG relative weight) are more meaningful than unadjusted LOS comparisons.

Acuity-Based Staffing Models

Acuity-based staffing models use quantified patient care requirements — measured through tools like the RAFAELA, PAONCIL, or hospital-proprietary nursing workload instruments — to determine appropriate nurse-to-patient ratios dynamically, rather than applying fixed ratio mandates. Acuity-based staffing analytics track shift-level workload distribution and identify chronic understaffing patterns that drive nurse burnout and patient safety incidents.