![]() ![]() In people where COVID- 19 has infected their lungs, these samples from the nose/throat can be negative and a deeper sample like sputum is needed to detect the virus. Throat/nasal swabs, and gargles/saliva may not have as much virus in them (so they would give a positive test with a higher Ct value). Nasopharyngeal swabs (those that go deep into the nose to swab the back of the upper throat) are the most sensitive specimen type for people who do not need admission to hospital. Ct values are affected by the type of the sample taken from the person ![]() Laboratories across the country have seen many cases where the person is tested early during their course of infection and the initial sample had a very high Ct value ~35 (low virus RNA concentration) and the following day the Ct was approximately 14 (high virus RNA concentration). >30 (low viral load), then to a lower Ct value (increased viral load) dramatically within a couple of days. A person with an initially negative result may progress to give a test with a high Ct value i.e. Factors to consider when interpreting Ct values Ct values will depend on the stage of infectionīetween exposure to the virus and symptom onset (e.g., incubation or pre-symptomatic period), the amount of virus in a person’s sample can be initially too low to be detectable (negative). But we do not know how much virus is actually required to cause an infection in someone and there are other important factors that may influence infectiousness, including the health of the person exposed and the type of exposure that has happened. A recent study which followed patients who were symptomatic but did not require hospitalization showed that those with higher viral loads (lower Cts) infected a higher proportion of their immediate contacts. Many believe that with low viral RNA copy numbers (high Ct value) the virus is not likely to be transmitted. So just because one can’t grow the virus in a laboratory that does not mean that it won’t transmit. However, the cells used in the laboratory to grow the virus are different from the cells in the back of the throat and nose (nasopharynx) or the lungs in people. There is good evidence that when more than 35 cycles are required to detect virus, the virus concentration is so low that it is unlikely to grow the virus in the laboratory. PCR tests cannot distinguish viral genomic material coming from intact viral particles in persons who are infectious or viral particle fragments that are present in individuals who have recovered. It is not possible to directly translate a Ct value into degree or duration of infectiousness.Ī person is deemed infectious if they shed virus particles that are intact and able to go on to infect others. Ct values and infectiousnessĪ frequent question is whether Ct values can help determine whether an individual is infectious or not. ![]() This can be complex and typically requires consultation between health care providers and laboratory specialists. Ct values can be used to monitor changes in the amount of virus present in a person’s samples over time. In certain circumstances, such as in patients with compromised immune systems, samples may need to be retested following recovery from COVID-19. It ensures that the PCR test is correctly detecting the presence of the virus and not false signals. This is defined by the manufacturer of the test or the laboratory during the validation process. The Ct value is the cut-off that calls a test positive. In addition, some molecular assays are based on other technologies (e.g., flow cytometry), and hence, do not provide Ct values. Not all commercial real-time PCR assays provide Ct values or amplification curves for viewing by the user. So a low Ct value corresponds to a high viral load, while a high Ct value corresponds to a low viral load.įor an example of a real-time amplification curve on a logarithmic scale, see Figure 1 in Public Health Ontario: An Overview of Cycle Threshold Values and their Role in SARS-CoV-2 Real-Time PCR Test Interpretation.Ĭurves can also be viewed on a linear scale, which will look different but does not change the Ct interpretation. The less RNA present in the clinical sample, the more cycles are required. The more RNA that is present in the patient sample, the fewer cycles are required for the signal to reach the detection threshold (low Ct value). The number of amplification cycles required to create enough copies of the viral RNA to be detected is called the cycle threshold or Ct value. The amount of genetic material doubles after each cycle. Most tests that detect the ribonucleic acid (RNA) or genetic fingerprint of the virus that causes COVID-19 (e.g., a polymerase chain reaction, or PCR test) use a process where specific bits of the genetic fingerprint are amplified using a temperature cycling reaction that repeats up to 45 times. Factors to consider when interpreting Ct values. ![]()
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