The Quick COVID-19 Severity Index (qCSI) calculator evaluates the risk of severe illness in COVID-19 patients based on three key health indicators. Users provide data for respiratory rate (in breaths per minute), pulse oximetry (as a percentage), and oxygen flow rate (in liters per minute). Each input is matched against predefined categories, each associated with a score, which collectively form the assessment.

The scoring is as follows:

- Respiratory Rate, breaths/min: 
  - <= 22: 0 points
  - 23-28: 1 point
  - > 28: 2 points

- Pulse Oximetry, %:
  - > 92%: 0 points
  - 89-92%: 2 points
  - <= 88%: 5 points

- Oxygen Flow Rate, L/min:
  - <= 2: 0 points
  - 3-4: 4 points
  - 5-6: 5 points

The total score is the sum of the individual scores from the above categories. Based on the cumulative score, the risk of critical illness within 24 hours is interpreted as follows:

- A score of 0 to 3 indicates a 4% risk of critical illness.
- A score of 4 to 6 indicates a 30% risk.
- A score of 7 to 9 indicates a 44% risk.
- A score of 10 or more indicates a 57% risk.

The Quick COVID-19 Severity Index (qCSI) is a clinical calculator designed specifically for patients with confirmed or suspected COVID-19 infection. The qCSI is used to predict the risk of developing severe respiratory disease, including the need for intubation or high-flow nasal cannula oxygen therapy. It is particularly useful in emergency department settings, where it can help clinicians triage patients and make decisions about hospital admission. The qCSI is based on three parameters: patient age, respiratory rate, and oxygen saturation. It does not apply to patients under the age of 18 or those with chronic respiratory diseases, as these groups were not included in the original study that validated the qCSI.