Key points for laboratory and imaging findings

Co-infection and secondary bacterial infection
Viral co-infection incidence varies in different case series (0%–19%)27,29
Combined bacterial and viral infection is rare in COVID-19 patients3,810
Secondary bacterial infection is not uncommon and leads to significant morbidity and mortality, especially in the elderly4,9,16
Detectable in 2 to 4 hours, peaks at 12 to 24 hours, and has a half-life of 25 to 30 hours
Levels are normal (< 0.5 μg/L) in COVID-19 patients with mild disease and may be elevated (≥ 0.5 μg/L) in patients with severe disease10,14
Elevated levels correlate with a nearly 5-fold higher risk of severe SARS-CoV-2 infection17
Elevated levels are not specific to bacterial infection because they can also be raised in patients with acute respiratory distress syndrome, end-stage renal disease, cardiogenic shock, and multiorgan failure18
A normal level makes bacterial infection less likely and can guide antibiotic discontinuation19,20
In bacterial infection, levels may be less affected by IL-6 inhibitors than is C-reactive protein (CRP)2123
CRP, erythrocyte sedimentation rate (ESR)
CRP and ESR are nonspecific inflammatory markers. Both are generally elevated in COVID-19 and are therefore not helpful in differentiating it from bacterial infection
Tocilizumab rapidly reduces CRP and leukocytosis and may suppress fever2426
Typical radiographic features of COVID-19
Chest radiography: bilateral, peripheral, lower-zone predominant air-space disease27
Computed tomography: bilateral, predominantly peripheral ground-glass opacities, crazy paving, and consolidation28; findings vary based on stage or phase of the disease
Typical radiographic features of bacterial pneumonia
Chest radiography: lobar or segmental air-space opacification ± air bronchograms
Computed tomography: segmental or lobar focal dense consolidation with or without ground-glass opacities