RT-PCR test is conducted to detect presence of sars CoV2 genetic material in the nasopharyngeal swab of a person . If the viral genetic material is present then the concerned person is designated as COVID-19 positive irrespective of whether the person is symptomatic or asymptomatic. My question: Is RT-PCR test a definite proof of the presence of live virus or it merely marks the presence of viral RNA (may or may not be from live virus) in the test sample?
Dear Mousumi Basu This test is effective when the virus is present in sufficient quantity. At the beginning of the disease 1-5 days, he will not give an accurate diagnosis of the virus. In severe form, after 7-10 days, the virus is concentrated in the patient's lungs. Samples should be taken with sputum of the lungs. In general, there is still no test system that 100 percent accurately diagnoses the COVID-19. Regards, Sergey Viktorovich Pushkin
Pushkin Sergey Viktorovich RT PCR is also effective providing positive result even a single SARS CoV 2 present in a particular sample. No need to occurrence of its sufficient quantity. Because the system amplify the DNA sequence to 35 folds in this case after the action of RT.
Dr. Naskar, I know the very test and agree with you that RT PCR can detect the presence of genetic material if it is present in minute quantity also. But my specific question was :
Can RT PCR be an infallible test for detection of viable virus ?
absolutely not.
- In the hours (+ -4 days according to the studies that I was able to consult) which follow a contamination the reliable RT PCRs (less than 35 multiplication cycles) are negative ... And there is live virus.
-When there is live virus, there is a possibility of contagion.
In the days following recovery, patients are no longer contagious (+ - 10 days after the onset of symptoms = + - 16 days after contamination). There is therefore no longer a living virus. However, RT PCRs remain positive well beyond. 50% positivity with 25 cycles on the 21st day after the onset of symptoms, i.e. + - 27 days after contamination.
This raises the whole question of the usefulness of RT PCRs.
- On the first day after a high risk of contamination (certain little protected contact with a confirmed patient): useful individually ... But the cases of direct contamination by a known contact are a large minority -> little impact in epidemiological management .
- When symptoms appear: useful for the choice of treatment, isolation, prevention of contact.
- After healing (which would represent the vast majority of tests carried out): totally useless: if there were symptoms they should have been carried out earlier. If there were no symptoms, there is absolutely no way of determining the time of contamination. Imagine that we will find the needle in the haystack, and utopian.
Their results are therefore totally useless both for the group and for the person being healed.
Regards.
It's true that Ct value indicates presence of Nucleic acid required for amplification irrespective of live or dead. Initially they are usually live and may spread but later on they are usually dead and unable to spread. Recently observed that initial high load becomes negative by use of iv Remdesevir within 3 days of treatment indicated by Ct value of 38. Even without antiviral treatment, Ct value will become negative but it will take longer duration, usually after 2 to 3 weeks from the date of infection. Presence of live virus that remains in early phase of disease is an important determinant for spreading. Later on when virus becomes dead, it doesn't spread although Ct may show some positivity.
Truly speaking, RT-PCR can't differentiate between live & dead fragments of nucleic acid. Only viral culture can differentiate that. Hence, interpretation may be done accordingly in relation to time of entry of virus in a human body.
I agree with you Dr. Sarkar on the fact that RT - PCR can detect presence of nucleic acid. But it doesn't confirm viability of the virus. Thank you for your explanation.
PCR targets viral DNA/RNA.....the sample for PCR means genetic material...I dont feel it makes any difference between live or dead...Additionally how much it is meaningful to label a virus as dead or live....