Objective: We aimed at identifying baseline predictive factors for emergency department (ED) readmission, with hospitalization/death, in COVID-19 patients previously discharged from the ED. We also developed a disease progression velocity index.

Design and setting: Retrospective cohort study of prospectively collected data. The charts of consecutive COVID-19 patients discharged from the Reggio Emilia (Italy) ED (March 2 - March 31, 2020) were retrospectively examined. Clinical, laboratory, and computed tomography (CT) findings at first ED admission were tested as predictive factors using multivariable logistic models. We divided CT extension by days from symptom onset to build a synthetic velocity index.

Participants: 450 patients discharged from the ED with diagnosis of COVID-19.

Main outcome measure: ED readmission within 14 days, followed by hospitalization/death.

Results: Of the discharged patients, 84 (18.7%) were readmitted to the ED and 61 (13.6%) were hospitalized and 10 (2.2%) died. Age (OR=1.05; 95% CI 1.03-1.08), Charlson Comorbidity Index 3 vs 0 (OR=11.61; 95% CI 1.76-76.58), days from symptom onset (OR for one day increase=0.81; 95% CI 0.73-0.90), and CT extension (OR for 1% increase=1.03; 95% CI 1.01-1.06) were associated in a multivariable model for readmission with hospitalization/death. A 2-day lag velocity index was a strong predictor (OR for unit increase=1.21, 95% CI 1.08-1.36); the model including this index resulted in less information loss.

Conclusions: A velocity index combining CT extension and days from symptom onset predicts disease progression in COVID-19 patients. For example, a 20% CT extension 3 days after symptom onset has the same risk as does 50% after 10 days.

Age and sex are standard parameters routinely collected for all patients. Charlson index was calculated based on hospital admissions in the previous 10 years. Date of COVID-19-related admission to the ED, date of symptom onset, diagnosis, hospitalization, and death, were retrieved from the COVID-19 Surveillance Registry, coordinated by the National Institute of Health and implemented in each Local Health Authority. Radiologist completed a routine CT report that included the extension of pulmonary lesions using a visual scoring system (< 20%, 20-39%, 40-59%, ≥ 60%). We built a synthetic indicator of the velocity of disease progression at ED admission, ie, the ratio of CT involvement and days from symptom onset to first ED admission, with 2 days lag and we approximate it without decimals.

Then we collapse the dataset for these varaibles and report the frequency in the "freq" variable.

There are 14 missing in extension of pulmonary lesions and consequentially in the velocity index.