Key points for laboratory and imaging findings
| Co-infection and secondary bacterial infection |
| Viral co-infection incidence varies in different case series (0%–19%)2–7,29 |
| Combined bacterial and viral infection is rare in COVID-19 patients3,8–10 |
| Secondary bacterial infection is not uncommon and leads to significant morbidity and mortality, especially in the elderly4,9,16 |
| Procalcitonin |
| 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)21–23 |
| 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 fever24–26 |
| 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 |