After PCR has been completed, a method called electrophoresis can be used to check the quantity and size of the DNA fragments produced. Related Content:. What is DNA replication? What is DNA? What is a gene? What is DNA sequencing? How helpful was this page? What's the main reason for your rating? Which of these best describes your occupation? What is the first part of your school's postcode? How has the site influenced you or others? Thankyou, we value your feedback! The baseline and calibration allow the scientist to interpret the results.
In this respect, the CEBM writes:. What does viral culture tell about PCR positives? A PCR test might find the virus it was looking for. This results in a PCR positive, but a crucial question remains: is this virus active, i. The PCR alone cannot answer this question. The CEBM explains why culturing the virus is needed to answer this question:. That is, if the PCR detects the virus in the human sample, this detection might correspond to a virus that is now incapable of infecting cells and reproducing.
Biologists can tell if the virus is infectious by injecting it into cells culture cells. If these cells are not affected by the virus and the virus does not reproduce in them, then the PCR test found a virus that is no longer active.
But this is not the only possibility. We want to focus on the CEBM argument that depends on viral culture. What did Tom Jefferson et al. What does this mean? Explanation of the experiment that shows whether a virus is still infective Imagine that a virus enters your body.
In a few months it might not do anything to you anymore. It might not do anything to your cells virulence , and it might also lack the capacity to move into another person infectivity when you speak or sneeze.
It is also possible that this virus simply never did anything to you and lacked infectivity from the very beginning. But traces of the virus might still be present in the person. In this case, the virus is present but inactive.
So how do you know if the virus is active? You do the PCR. If by injecting that virus into culture cells, the virus is not able to reproduce in the cells, that virus cannot infect anybody any longer. This means that even if you are a PCR positive, you are no longer contagious, that is, the virus in you is no longer active.
The virus cannot be transmitted when cell culture shows that the virus is not infective. See next. Is there evidence that someone is infectious after PCR results? Tom Jefferson et al. We recall that currently they governments hardly look for symptoms in people. Positives are called PCR Positive asymptomatic if they present no symptoms.
In the article the authors say:. There is some evidence of a relationship between the time from collection of a specimen to test, symptom severity and the chances that someone is infectious.
One of the studies we found Bullard et al investigated viral culture in samples from a group of patients and compared the results with PCR testing data and time of their symptom onset.
Because PCR positives have not been correlated to the growth of the virus in culture. They continue to explain why this correlation is not possible:. Can successive tests on the same person give contradictory results? That a PCR test gives positive or negative depends on how the experiment is conducted. The authors claim:. The higher the viral concentration the lower amplification cycles are necessary. Some people might give positive after running the PCR test with a high threshold and others with a low threshold.
The threshold alone might or might not tell whether someone carries infective viral RNA. How long can an inactive virus remain in a body? This is inconclusive since PCR positives to viral culture studies are lacking and cycle thresholds should also be considered.
See above. The way in which the experiment is carried out however, matters. This is because one might be PCR Positive long after the virus is no longer active. The authors briefly explain why:. The authors show a figure figure 2 where it is noted that the presence and detection of viral RNA by PCR does not imply that the virus is infectious or virulent any longer.
PCR kits for SARS Cov2 manufacturers and asymptomatic PCR positives on asymptomatic people should be treated with care since it is possible that the asymptomatic people are not infectious. The entire cycling process of PCR is automated and can be completed in just a few hours. It is directed by a machine called a thermocycler, which is programmed to alter the temperature of the reaction every few minutes to allow DNA denaturing and synthesis.
What is PCR? What is PCR used for? How does PCR work?
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