Home Biomarkers Cycle Threshold RDRP

Cycle Threshold RDRP

COVID Biomarker

Sample Needed

Collection Type: Nasal or throat swab

Body System

Related System: COVID

Overview

The Cycle Threshold (Ct) for the RdRp (RNA-dependent RNA polymerase) target is a numeric result from a SARS‑CoV‑2 RT‑PCR test that indicates how many amplification cycles were required to detect viral RNA specific to the RdRp gene. Lower Ct values mean more viral RNA in the specimen (higher viral load); higher Ct values mean less RNA. The test is used to detect active COVID‑19 infection and can help infer infectiousness and stage of illness. Indications include fever, cough, sore throat, loss of taste/smell, shortness of breath, or known exposure. Ct values can vary with time since symptom onset, specimen type and quality, assay platform, and patient factors (age, comorbidities, vaccination status); gender has minimal direct effect.

People also test for

Test Preparation

No special preparation is required

Why Do I Need This Test

Which profile is the test included in: COVID profile (SARS‑CoV‑2 RT‑PCR, RdRp target).
Symptoms indicating need: fever, respiratory symptoms, anosmia, sudden malaise, or known exposure.
Conditions diagnosed/monitored: acute SARS‑CoV‑2 infection, viral clearance, monitoring response to therapy.
Reasons for abnormal values: early/late infection, poor sampling, low viral load, recent vaccination or antiviral therapy.
Biological meaning of abnormal values: low Ct = high viral load; high Ct or undetectable = low/absent viral RNA.
Behaviors/lifestyle causing abnormal values: recent exposure, lack of isolation, timing of test after symptoms, incomplete swabbing.
Family history: close household contact with COVID‑19 increases pretest probability and need for testing.

Run our symptom checker to see if this test is right for you

Symptom Checker

Understanding Test Results

Ct values are interpreted relative to assay cutoffs.
Typical interpretive ranges: Ct 20 cycles very high viral load, usually early/active infection and high infectiousness; Ct 20–30 moderate to high viral load, likely infectious; Ct 30–35 low to moderate viral RNA, decreasing infectiousness; Ct 35–40 very low viral RNA, may represent late infection, residual RNA, or low-level infection; >40 or undetermined negative (no detectable viral RNA).
Single Ct values must be correlated with symptoms, timing (days since onset), specimen type, and lab assay; repeat testing or clinical correlation is recommended for borderline/discordant results.

Normal Range

No detectable Ct (undetermined) OR Ct 40 cycles

FAQs

Q: What is the threshold cycle in qPCR?

A: The threshold cycle (Ct) in qPCR is the PCR cycle number at which the amplified product’s fluorescence first exceeds a defined threshold above background. It marks when exponential amplification becomes detectable and is inversely proportional to the initial amount of target nucleic acid—lower Ct means more starting template. Ct values are used for relative or absolute quantification with appropriate controls and calibration.

Q: What is the threshold value in CT?

A: The CT threshold value refers to a cutoff in Hounsfield units (HU) used to distinguish tissues. The HU scale: air -1000 HU, fat -100 to -50 HU, water 0 HU, soft tissue +20 to +100 HU, and bone +300 to +1000 HU. Thresholds are chosen for segmentation or windowing to isolate structures or pathologies.

Q: How to set threshold for PCR?

A: Set the qPCR fluorescence threshold within the exponential phase above baseline noise so Ct falls in the linear amplification range. Use baseline-corrected data, include no‑template and positive controls, and apply the same thresholding method across runs. Verify automatic thresholds visually, adjust to exclude background or plateau signal, and define predetermined Ct cutoffs for positivity and retesting.

Q: What is the threshold determination in qPCR?

A: Threshold determination in qPCR defines the fluorescence level used to calculate the cycle threshold (Ct/Cq). The threshold is placed above baseline noise and within the exponential amplification phase so the crossing point reflects true template amplification. It can be set manually or automatically (fixed fluorescence or proportional to baseline variability). Proper thresholding ensures accurate, consistent quantification across samples and runs.

Q: What is the threshold value?

A: A threshold value is the specific cutoff or point on a measurement scale used to distinguish normal from abnormal or to trigger an action (diagnosis, treatment, or public-health response). Thresholds are set from clinical evidence, balancing benefits, harms, sensitivity and specificity, and may vary by population, age, or risk factors. Guidelines specify threshold values for particular conditions.

Q: What does a low CT value mean in PCR results?

A: A low CT (cycle threshold) value means the PCR test detected the target genetic material after few amplification cycles, indicating a higher amount of viral or target nucleic acid in the sample. It often correlates with higher viral load and greater likelihood of infectiousness, but is influenced by specimen quality, timing of testing, and assay differences; CT values are semi‑quantitative and not directly comparable across labs.