Which SARS-CoV-2 gene targets are used in COVID-19 RT-PCR diagnostic assays?

WHO-recommended RT-PCR assays target the N (nucleocapsid), E (envelope), RdRp (RNA-dependent RNA polymerase), and ORF1ab genes of SARS-CoV-2. Multi-target strategies improve sensitivity and provide redundancy against emerging variants with mutations in primer binding sites.

SARS-CoV-2 Genome and Diagnostic Targets

SARS-CoV-2 is a positive-sense RNA virus with a ~30 kb genome. Key diagnostic targets: N gene (nucleocapsid, most abundant transcript, used by US CDC), E gene (envelope, WHO/Charité assay), RdRp (RNA polymerase, highly conserved among betacoronaviruses), ORF1ab (replicase polyprotein, Chinese CDC assay), and S gene (spike, used for variant genotyping).

WHO-Recommended Primer and Probe Sequences

Charité E Gene: E_Sarbeco_F1: ACAGGTACGTTAATAGTTAATAGCGT, E_Sarbeco_R2: ATATTGCAGCAGTACGCACACA, Probe: FAM-ACACTAGCCATCCTTACTGCGCTTCG-BHQ1

US CDC N Gene: 2019-nCoV_N1-F: GACCCCAAAATCAGCGAAAT, N1-R: TCTGGTTACTGCCAGTTGAATCTG, N1-P: FAM-ACCCCGCATTACGTTTGGTGGACC-BHQ1

The R and Y degenerate bases in RdRp primers account for sequence variation between SARS-CoV and SARS-CoV-2 (see degenerate primer design).

Primer Design for Emerging Variants

Omicron sub-lineages carry mutations in N1/N2 primer binding regions, potentially reducing sensitivity. The CDC recommends both N1 and N2 targets for redundancy. S gene target failure (SGTF) from the ΔH69/V70 deletion was used for Alpha and Omicron BA.1 surveillance. ORF1ab mutations are rare due to functional constraints. Design strategy: use a dual-target approach (e.g., N + ORF1ab) and re-validate primers periodically.

Multiplex RT-PCR for SARS-CoV-2

Most diagnostic assays use triplex RT-PCR: Target 1 (FAM), Target 2 (VIC/HEX), and internal control RNase P (Cy5). Balance primer ratios to avoid dominant target suppression (see multiplex optimisation). Include ROX as passive reference. Use 2-step cycling: 95°C for 5 s, 60°C for 30 s, for 40–45 cycles.

RT-PCR Troubleshooting

  • False negatives: Use a second independent target. Collect a new sample from a different site.
  • Weak positive in asymptomatic: May be low-level infection or residual RNA. Repeat on a fresh sample.
  • No internal control: Failed extraction or inhibition. Repeat extraction.
  • High Ct variation: RT enzyme is viscous. Pre-mix components before adding to plate.

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