COVID Biomarker
Collection Type: Nasal or throat swab
Related System: COVID
The Cycle Threshold (Ct) for the E-gene is a value produced by real-time RT‑PCR assays that detect SARS‑CoV‑2 RNA. It represents the number of amplification cycles required for fluorescent signal from the E (envelope) gene target to cross a predefined threshold; lower Ct more viral RNA in the sample (higher viral load). This test is used to detect active COVID‑19 infection and to estimate relative viral burden. It is ordered when patients have symptoms such as fever, cough, sore throat, shortness of breath, loss of taste/smell, or after known exposure. Ct values vary with timing since symptom onset (lowest early), specimen type/quality, and to a lesser extent with age, immune status, vaccination, or underlying illness; gender differences are minimal.
Run our symptom checker to see if this test is right for you
Symptom CheckerCt >40 cycles (Not detected Negative)
Q: What is the E gene in a COVID test?
A: The E gene (envelope gene) encodes the small membrane envelope protein of SARS‑CoV‑2 and is commonly targeted in PCR screening assays. Its detection indicates coronavirus RNA presence and is sensitive but can cross‑react with related sarbecoviruses, so positive E‑gene results are often confirmed using more specific targets (e.g., N, RdRp or S genes) or sequencing to verify SARS‑CoV‑2 infection.
Q: Is a high proportion of low cycle threshold value as an early indicator of COVID-19 surge?
A: Yes a rising share of low cycle-threshold (Ct) PCR results (which indicate higher viral load) can precede and signal increased community transmission or a forthcoming surge. However, Ct values vary by assay, swab timing and lab practices, so they are an imperfect marker: use aggregated, consistently measured Ct trends alongside case rates, hospitalisations and wastewater surveillance before concluding a true surge.
Q: How accurate is the E gene RT-PCR test?
A: The E‑gene RT‑PCR for SARS‑CoV‑2 is highly accurate—offering strong sensitivity and specificity when performed on good respiratory samples and interpreted with cycle‑threshold (Ct) values. However it’s not perfect: false negatives can occur with low viral load, poor sampling, or late testing; false positives are rare but possible from contamination or single‑target assays. Confirmation with additional gene targets improves reliability.
Q: What does CT value mean in COVID testing?
A: CT value (cycle threshold) in COVID PCR testing is the number of amplification cycles needed to detect viral genetic material. Lower CT means more viral RNA (higher viral load); higher CT means less viral RNA. CT values are semi-quantitative, vary by test and sample quality, and alone cannot definitively determine infectiousness; clinical context and repeat testing guide interpretation.
Q: What is a good CT value for COVID?
A: A low PCR cycle threshold (Ct) indicates higher viral load. Typical interpretations: Ct <25 suggests high viral load and likely infectious; 25–30 is moderate; 30–35 is low; Ct >35 often reflects very low viral RNA or non‑viable virus. Ct values vary by assay and timing, so don’t use Ct alone—interpret with clinical context, symptoms, and repeat or alternative testing.
Q: Is a low CT value good or bad?
A: A low CT value is generally considered bad because it means the PCR detected viral genetic material after fewer amplification cycles, implying a higher viral load and greater infectiousness. However, CT values vary by test, sample quality and timing, so a single CT number shouldn’t be used alone to judge disease severity or contagiousness; clinical context and repeat testing are important.