Do Old COVID-19 Tests Work for New Strains?
Yes, old COVID-19 tests continue to work effectively for detecting new strains, as both PCR and antigen tests target conserved regions of the SARS-CoV-2 genome that remain present across all variants. 1
PCR Testing Remains Reliable Across Variants
RT-PCR tests detect specific genetic material (RNA) of SARS-CoV-2 by targeting conserved regions of the viral genome that are present across all variants, making them the gold standard for diagnosis regardless of which strain is circulating 1
Standard real-time PCR tests can confirm the presence of SARS-CoV-2 but cannot distinguish between specific variants (Alpha, Delta, Omicron, etc.), though this does not impair their ability to detect infection 1
RT-PCR targets multiple genes including RNA-dependent RNA polymerase (RdRp), open reading frame (ORF1), Envelope (E), and nucleocapsid (N) genes, providing redundancy even if mutations occur in one target region 2
Antigen Tests Continue to Function Despite Mutations
Antigen tests maintain high specificity across variants, and a positive result does not require routine confirmation with PCR, according to the Infectious Diseases Society of America 2
For symptomatic patients tested within 5 days of symptom onset, antigen tests demonstrate 89% sensitivity and nearly 100% specificity when compared to nucleic acid amplification testing as the reference standard 2
The Centers for Disease Control and Prevention recommends testing symptomatic individuals within 5 days of symptom onset for optimal performance of both PCR and antigen tests 3
Important Caveats About Variant Detection
While diagnostic tests can detect all variants, monitoring the performance of antigen tests for new-variant COVID-19 remains critical as new variants emerge 2
Mutations have been documented on essentially all current COVID-19 diagnostic targets, with the nucleocapsid (N) gene showing the most mutations and being one of the most non-conservative genes in the SARS-CoV-2 genome 4
Despite these mutations, the conserved regions targeted by most diagnostic tests remain sufficiently stable to maintain test accuracy across variants 1
Clinical Implications for Testing Strategy
A positive PCR test should be interpreted as confirmation of COVID-19 infection with a currently circulating variant, requiring appropriate isolation measures 1
Negative antigen results must be confirmed with standard NAAT if clinical suspicion for COVID-19 remains moderate or high, as per CDC recommendations 3
Never assume a negative antigen test rules out infection in symptomatic patients with high clinical suspicion—always confirm with NAAT 3
The sensitivity of PCR testing varies by specimen type, with nasopharyngeal specimens providing superior pathogen detection compared to throat swabs 3