Which tests should be performed to assess the severity of a COVID‑19 infection?

Assessment of COVID-19 Severity: Diagnostic Testing Algorithm

For patients with confirmed or suspected COVID-19, perform RT-PCR nasopharyngeal swab testing combined with baseline laboratory markers (lymphocyte count, CRP, D-dimer, LDH, ferritin) and chest imaging (CT scan preferred, chest X-ray if CT unavailable) to assess disease severity and predict outcomes. 1

Initial Diagnostic Confirmation

• RT-PCR nasopharyngeal swab is the gold standard for COVID-19 diagnosis, though sensitivity varies with timing and sample collection technique 1
• Nasopharyngeal samples are superior to oropharyngeal samples in early disease stages 1
• If initial RT-PCR is negative but clinical suspicion remains high, repeat testing combined with chest imaging is recommended 1

Laboratory Markers for Severity Assessment

Key laboratory tests that predict severe disease and poor outcomes include:

• Lymphocyte count <0.8-1.0 × 10⁹/L indicates higher severity 1
• C-reactive protein (CRP) >90 mg/L correlates with worse pulmonary involvement 1, 2
• D-dimer elevation predicts thrombotic complications and severe disease 1
• Lactate dehydrogenase (LDH) elevation indicates tissue damage and severity 1
• Ferritin elevation correlates with inflammatory burden 1, 3
• White blood cell count >10,000/μL or leucopenia both indicate severity 1, 2
• Elevated aspartate aminotransferase (AST), troponin I, and creatinine suggest multi-organ involvement 1

Severity Classification Based on Clinical Parameters

Use the WHO-China Joint Mission criteria for standardized severity definitions: 1

• Mild-to-moderate: Non-pneumonia or mild pneumonia 1
• Severe: Respiratory rate ≥25 breaths/min, oxygen saturation ≤93% at rest, or PaO₂/FiO₂ ratio <300 mmHg 1, 2
• Critical: Respiratory failure requiring mechanical ventilation, septic shock, or multi-organ dysfunction requiring ICU care 1

Chest Imaging for Severity Assessment

CT Chest (Preferred Modality)

• Chest CT is the most accurate radiological tool for confirming diagnosis and assessing pneumonia severity, particularly in uncertain cases with negative RT-PCR 1
• Bilateral ground-glass opacities, consolidation, and interlobular septal thickening are characteristic findings 1
• Semi-quantitative CT severity scoring (0-25 scale) predicts ICU admission, mortality, and hospitalization length 2
• CT findings correlate with persistent symptoms and long-term outcomes 4
• Obtain non-contrast baseline CT for initial assessment 1

Chest X-Ray (Alternative When CT Unavailable)

• Chest X-ray sensitivity is only 43.5-69% compared to CT, but remains useful when CT is unavailable 1, 3
• Bilateral interstitial patterns and ground-glass opacities are common findings 1
• CXR severity scores (5-point or 12-point scales) correlate with dyspnea, oxygen saturation, CRP, ferritin, D-dimer, and LDH levels 3
• Lower lobe involvement is more common than upper lobe 3
• Obtain both PA and lateral views when possible 1

Lung Ultrasound (Point-of-Care Option)

• Lung ultrasound can be used as first-line screening when CT is unavailable or contraindicated, particularly by clinicians with excellent POCUS skills 1
• Pathological findings include irregular pleural line, B-lines (vertical artifacts), white lung appearance, and subpleural consolidations 1
• Lung US differentiates low-risk patients (negative US, clinically stable) from high-risk patients requiring urgent CT 1
• Sensitivity of 92-93% for detecting consolidation and pleural effusions 1

Clinical Predictors Requiring Immediate Severity Assessment

Obtain comprehensive testing immediately if patient presents with: 1, 2

• Respiratory rate ≥25 breaths/min 1, 2
• Oxygen saturation ≤93% at rest 1, 2
• Dyspnea, particularly at rest 1
• Anosmia (associated with higher CT severity scores) 2
• Multiple comorbidities (diabetes, hypertension, cardiovascular disease, chronic lung disease) 1
• Age >60 years 1

Critical Pitfalls to Avoid

• Do not rely on single negative RT-PCR to exclude COVID-19 when clinical suspicion is high; repeat testing and add chest imaging 1
• Do not assume normal chest X-ray excludes pneumonia; CXR can be normal early in disease and has lower sensitivity than CT 1, 3
• Do not delay severity assessment in patients with risk factors even if initial symptoms appear mild, as rapid progression can occur 1
• Avoid using CT as a screening tool alone without combining with RT-PCR and clinical data 1
• Monitor D-dimer closely as most severe patients develop coagulation disorders and microthrombotic formation 1