Briefly discontinue supplemental oxygen for 1–2 minutes and measure SpO2 — most stable patients tolerate this without significant desaturation. If the patient desaturates rapidly to below 90% or becomes symptomatic within seconds of removing O2, do not continue the test — record the O2 requirement as the primary severity indicator and note SpO2 cannot be safely assessed on room air. This is itself a high-risk sign.

QCSI was derived and validated specifically in COVID-19 patients. While the underlying physiological variables (O2 requirement, RR, SpO2) are relevant in other hypoxemic respiratory illnesses, its calibrated risk thresholds (1.4% vs 46.4% ICU admission) are based on COVID-19 cohorts. Apply with caution in other respiratory illnesses and use NEWS2 or CURB-65 for non-COVID respiratory triage.

QCSI 4–6 corresponds to 11.7% ICU admission risk — a meaningful risk that warrants strong consideration of hospitalization or at minimum extended observation. Key factors supporting admission: O2 requirement ≥2 L/min, age >60, significant comorbidities (diabetes, obesity, immunosuppression, heart failure), social circumstances that preclude close monitoring at home, or inability to reliably monitor SpO2 at home.

Yes. Serial QCSI assessment (every 4–8 hours or with clinical status change) tracks respiratory trajectory during hospitalization. An increasing QCSI despite treatment signals deterioration and should prompt consideration of escalating level of care, early respiratory therapy or critical care consult, and preparation for potential intubation.