What labs should be ordered for a COVID-19 patient?

Laboratory Tests for COVID-19 Patients

For COVID-19 patients, standard nucleic acid amplification testing (NAAT) is strongly recommended as the first-line diagnostic test, followed by a targeted panel of laboratory tests including complete blood count, inflammatory markers (CRP, procalcitonin), D-dimer, and coagulation studies to assess disease severity and guide management.

Diagnostic Testing

Initial Diagnosis

• Nucleic acid amplification testing (NAAT) is the gold standard for diagnosis 1, 2
  • If NAAT results would be delayed >24 hours, use rapid antigen testing instead
  • For optimal performance, testing should be performed within 5 days of symptom onset
  • Acceptable specimens (in order of preference):
    • Nasopharyngeal (NP) swab
    • Midturbinate (MT) swab
    • Combined anterior nasal (AN) plus oropharyngeal (OP) swab
    • Saliva or mouth gargle specimens

Specimen Collection Considerations

• Anterior nasal and midturbinate specimens can be self-collected or collected by healthcare providers 1
• For self-collection, either observed or unobserved collection is acceptable with proper instructions
• If clinical suspicion remains high after a negative antigen test, confirm with standard NAAT 1

Essential Laboratory Tests for COVID-19 Patients

Hematologic Parameters

• Complete blood count (CBC) 1, 3, 4
  • White blood cell count (WBC) - elevated WBC >10×10⁹/L associated with increased mortality
  • Absolute lymphocyte count (ALC) - lymphopenia common in severe disease
  • Absolute neutrophil count (ANC)
  • Neutrophil-to-lymphocyte ratio (NLR) - NLR >9 associated with increased mortality
  • Platelet count - thrombocytopenia (<150×10⁹/L) associated with increased mortality
  • Hemoglobin (Hb)

Inflammatory Markers

• C-reactive protein (CRP) - CRP >100 mg/L associated with increased mortality 1, 3, 5
• Procalcitonin (PCT) - useful for distinguishing bacterial from viral infection 1, 6
• Ferritin - levels >1000 ng/mL associated with increased mortality 3, 5
• Lactate dehydrogenase (LDH) - elevated in severe disease 5, 7
• Interleukin-6 (IL-6) - if available, elevated in severe disease 3

Coagulation Studies

• D-dimer - elevated in severe disease and associated with thrombotic complications 3, 5
• Prothrombin time (PT) 3
• Activated partial thromboplastin time (APTT) 3
• Fibrinogen - often elevated in COVID-19 7

Other Important Tests

• Renal function tests 6
  • Blood urea nitrogen (BUN)
  • Creatinine
• Liver function tests 6, 7
  • Aspartate aminotransferase (AST)
  • Alanine aminotransferase (ALT)
  • Gamma-glutamyl transferase (GGT)

Testing Algorithm Based on Disease Severity

For Mild-Moderate Outpatient Cases

1. NAAT or antigen test for diagnosis
2. Consider CBC, CRP if symptoms worsen

For Hospitalized Patients

1. NAAT for diagnosis (if not already confirmed)
2. Complete panel:
   • CBC with differential
   • CRP and/or procalcitonin
   • D-dimer
   • Ferritin
   • LDH
   • Renal and liver function tests
   • Coagulation studies (PT, APTT)

For ICU/Critical Patients

1. All tests for hospitalized patients
2. More frequent monitoring of:
   • CBC with differential (monitor NLR)
   • D-dimer
   • Ferritin
   • LDH
   • Procalcitonin (to detect secondary bacterial infections)

Clinical Pearls and Pitfalls

• Biomarker interpretation: No single biomarker should be used in isolation; interpret in clinical context 1
• Timing matters: Laboratory values change throughout disease course; trending is more valuable than single measurements 3, 5
• Avoid unnecessary testing: For asymptomatic or mildly symptomatic outpatients, extensive laboratory testing is not routinely recommended 1
• Repeat testing: For hospitalized patients, regular monitoring of key parameters (CBC, inflammatory markers) is important to detect disease progression 1, 5
• Coagulopathy monitoring: COVID-19 is associated with hypercoagulability; monitor coagulation parameters closely, especially in hospitalized patients 1
• Testing for viral clearance: Repeat NAAT is not recommended to determine the end of isolation period 2

By systematically monitoring these laboratory parameters, clinicians can better assess disease severity, predict complications, and guide therapeutic interventions for COVID-19 patients.