Are last year's COVID-19 (Coronavirus Disease 2019) tests effective against the new strain of the virus?

COVID-19 Tests and New Variants: Effectiveness of Last Year's Tests

Current COVID-19 tests remain effective for detecting new strains, but monitoring for variant-specific performance changes is essential, particularly for antigen tests that target the nucleocapsid protein. 1

Current Testing Landscape

Types of COVID-19 Tests

• Nucleic Acid Amplification Tests (NAATs):

  • RT-PCR remains the gold standard for diagnosis with highest sensitivity 1
  • Target genes include RNA-dependent RNA polymerase (RdRp), open reading frame (ORF1), Envelope (E), and nucleocapsid (N) genes 1
  • Laboratory-based tests typically have 6-8 hour turnaround time; rapid versions available in 45-60 minutes 1

• Antigen Tests:

  • Most detect nucleocapsid (N) protein 1
  • Lower sensitivity than NAATs but offer rapid results (15 minutes) 1
  • Optimal performance within first 5 days of symptoms 1
  • High specificity (positive results rarely need confirmation) 1

Performance with New Variants

The IDSA guidelines (2024) specifically address variant concerns:

1. Antigen Tests and Variants:

   • Most antigen tests detect the nucleocapsid protein, which has less variation than the spike protein 1
   • Performance was only slightly reduced with Omicron-BA.1 variant 1
   • Some tests may have reduced sensitivity with specific variants (e.g., Omicron-BA.2) 1
   • IDSA notes: "As new variants emerge, the performance of Ag tests may change. Therefore, monitoring the performance of Ag tests for diagnosis of new-variant COVID-19 is critical" 1

2. Molecular Tests (NAATs):

   • Generally more robust against variants due to multiple gene targets 1
   • False negatives can occur due to mutations in the RT-PCR target region 1
   • Remain the preferred option for highest sensitivity 1

Testing Strategy Recommendations

For Symptomatic Individuals:

1. First Choice: Standard NAAT (laboratory-based or rapid RT-PCR) 1

   • Highest sensitivity to detect infection
   • Preferred when results available within 24 hours

2. Alternative: Antigen testing 1

   • When NAAT results would be delayed >24 hours
   • Within first 5 days of symptoms for optimal performance
   • Negative results should be confirmed with NAAT if clinical suspicion remains high

Testing Algorithm for New Variants:

1. For highest accuracy: Use standard NAAT/RT-PCR testing
2. For rapid results: Antigen testing is acceptable with understanding of limitations
3. For negative antigen tests: Confirm with NAAT if symptoms are consistent with COVID-19

Important Caveats and Pitfalls

• False Negatives: Can occur with any test but more common with antigen tests (sensitivity ~89% for symptomatic patients within 5 days) 1

  • Causes include: improper sampling, low viral loads, mutations in target regions 1

• Timing Matters:

  • Antigen tests perform best within 5 days of symptom onset 1
  • Viral RNA detection decreases 10-20 days after diagnosis 1

• Variant-Specific Concerns:

  • Some variants may affect test performance, particularly antigen tests targeting specific proteins 1
  • Continuous monitoring of test performance against new variants is necessary 1

• Alternative Diagnoses:

  • Consider other respiratory viruses if COVID-19 tests are negative 1
  • Multiple testing may be needed if clinical suspicion remains high

The evidence strongly supports that while most current COVID-19 tests remain effective for detecting new variants, ongoing vigilance and monitoring of test performance is essential as the virus continues to evolve.