What laboratory tests are recommended for COVID-19 patients?

Recommended Laboratory Tests for COVID-19 Patients

For COVID-19 patients, a targeted panel of laboratory tests is recommended, including complete blood count with differential, inflammatory markers (C-reactive protein, ferritin, procalcitonin), and coagulation studies (D-dimer, fibrinogen), with additional cardiac markers for those with cardiopulmonary symptoms. 1

Core Laboratory Testing

Diagnostic Testing

• First-line diagnostic test: Nucleic acid amplification testing (NAAT) 1
  • Preferred specimen types (in order): nasopharyngeal swab, midturbinate swab, combined anterior nasal plus oropharyngeal swab, or saliva specimens
  • Optimal timing: within 5 days of symptom onset
  • Antigen testing can be used but has lower sensitivity; negative results with moderate/high clinical suspicion require NAAT confirmation

Essential Laboratory Panel

1. Complete Blood Count (CBC) with differential 1, 2

   • White blood cell count (WBC)
   • Absolute lymphocyte count (ALC) - often decreased in COVID-19 3
   • Absolute neutrophil count (ANC)
   • Neutrophil-to-lymphocyte ratio (NLR) - correlates with disease severity 4
   • Platelet count - may be decreased 3
   • Hemoglobin (Hb) 5
   • Red cell distribution width (RDW) - associated with disease severity 3

2. Inflammatory Markers 1, 2

   • C-reactive protein (CRP) - values >100 mg/L associated with increased mortality
   • Ferritin
   • Procalcitonin (PCT) - useful for distinguishing bacterial from viral infection
   • Erythrocyte sedimentation rate (ESR)
   • Interleukin-6 (IL-6) - if available

3. Coagulation Studies 2, 6

   • D-dimer - often elevated in severe disease
   • Fibrinogen - typically elevated in COVID-19
   • Prothrombin time (PT)/INR
   • Activated partial thromboplastin time (aPTT)

4. Additional Basic Testing 1, 2

   • Basic metabolic panel
   • Liver function tests (ALT, AST)
   • Serum albumin - lower levels associated with disease severity 3
   • Lactate dehydrogenase (LDH)

Specialized Testing Based on Presentation

For Patients with Cardiopulmonary Symptoms 2

• Cardiac Triad Testing:
  • Electrocardiogram (ECG)
  • Cardiac troponin (cTn) - high-sensitivity assay preferred
  • Echocardiogram
• Additional cardiac markers:
  • Brain natriuretic peptide (BNP) or NT-proBNP

For Patients with Suspected Coagulopathy 2, 6

• More comprehensive coagulation panel:
  • Antithrombin
  • Plasminogen
  • Plasminogen activator inhibitor-1 (PAI-1)
  • Factor V

Testing Frequency and Monitoring

• Hospitalized patients: Monitor CBC, inflammatory markers, and coagulation parameters at least twice daily 2
• Outpatients with mild symptoms: Extensive laboratory testing not routinely recommended 1
• Patients with worsening symptoms: Repeat testing to assess disease progression

Important Considerations

1. Test interpretation caveats:

   • No single biomarker should be used in isolation 1
   • Similar coagulation abnormalities may be present in non-COVID respiratory infections 7
   • NLR can be a simple, independent predictor of disease severity even in resource-limited settings 4

2. Common pitfalls to avoid:

   • Relying solely on normal WBC counts to rule out COVID-19 (differences between COVID-19 and non-COVID patients may be small) 3
   • Overinterpreting isolated abnormal values without clinical context
   • Unnecessary repeat testing for determining end of isolation period 1

3. Special populations:

   • Pregnant patients: May present with thrombocytopenia and low fibrinogen, differing from non-pregnant COVID-19 patients 2
   • Athletes: Cardiac testing recommended for those with cardiopulmonary symptoms before return to play 2

By following this structured approach to laboratory testing, clinicians can effectively diagnose, risk-stratify, and monitor COVID-19 patients while avoiding unnecessary testing in those with mild disease.