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COVID-19: Here's All You Need to Know About RT-PCR and Rapid Antigen Test

(Representational Photo: Shutterstock)

(Representational Photo: Shutterstock)

To detect the virus infection in human body we need to detect the soul of virus. There are methods or tests to detect nucleic acid (soul) i.e. RNA. Let’s solve the queries related to these tests

COVID is COrona VIrus Disease caused by SARS COV 2 Severe Acute Respiratory Syndrome COrona Virus 2. This name to virus is given because it causes severe and acute infection of lungs. First case detected in China in December 2019, so the name COVID-19 was given. The virus came earlier in 2002 in limited part of East Asia was named SARS COV 1. Virus can either be DNA (deoxy-ribo nucleic acid) or RNA (ribose nucleic acid) virus.

Corona virus is a RNA virus. Nucleic acid is genetic material of virus just like soul in human body. So to detect the virus infection in human body we need to detect the soul of virus. There are methods or tests to detect nucleic acid (soul) i.e. RNA. Let’s solve the queries related to these tests.

What are the methods available to detect COVID-19?

Two methods commonly used to detect COVID causing virus are RT-PCR and RAT (card test).

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What is RT-PCR?

RT-PCR means Reverse Transcriptase Polymerase Chain Reaction. Also known as Real Time PCR. This method is used to amplify (i.e. to make multiple copies of) RNA in the sample containing very small quantity of virus so as to detect the presence of virus in given sample accurately and in less time. Although the machine takes only 3-6 hours to give result but considering the huge number of samples, reporting may delay.

What does positive and negative reports mean?

  • True positive means test has detected the virus when virus is actually present in sample. (Test report positive, Virus present in sample).
  • False positive means test has detected virus wrongly when virus is actually not present in the sample (May happen due to impurity in sample.) (Test report positive, Virus not present in sample)
  • True negative means test has not detected any virus, in the sample actually having no virus. (Test report negative, Virus not present in sample)
  • False negative means test has not detected the virus even if sample have virus. (Test report negative, Virus present in sample)
  • So Positive RT-PCR report is considered as presence of virus but the negative report cannot surely say that the virus is not present. And hence a repeat RTPCR is suggested.

How sensitive RTPCR is?

Sensitivity means power of test to detect virus in sample having virus i.e. power of test to give true positive result.

Sensitivity of RT-PCR to detect SARS COV 2 in swab sample from nose and throat is around 85%. It means out of 100 samples, 85 samples give positive result. There are chances that 15 samples may have virus but not detected by RT-PCR (false negative). In such cases by repeating RT-PCR may detect the virus. Hence it recommended repeating RT-PCR second time in case first RT-PCR report is negative.

How specific RT-PCR is?

Specificity means power of test to reject (not detect) the virus when it is actually not present in sample i.e. power of test to give true negative result.

Specificity of RT-PCR to detect SARS COV 2 in swab sample from nose and throat is around 98%. It means that out 100 samples in only 2 samples it gives false positive result i.e. detect virus even in absence of virus in sample. So because RTPCR is having good specificity, positive results are considered as true positive and consider it having the disease. So it is not recommended to repeat the test when it comes positive.

What is CT value in RT-PCR report means?

CT means cycle threshold. The CT value refers to the number of cycles after which the virus can be detected. The lower the Ct value, the higher the viral load because the virus has been spotted after fewer cycles.

Is more CT value means the disease is more severe?

No. there is no correlation between CT value and severity of disease. CT value tells about the viral load. Less CT value means more viral load in patient and hence more infectivity i.e. the patient spread the disease more to other. 24 is the cut off value, Patients with CT value less than 24 can spread disease more as compare to patient with CT value more than 24.

What id RAT?

RAT means Rapid Antigen Test. Unlike RT-PCR which detects nucleic acid (RNA), RAT detects protein in the swab sample.

Advantage of RAT over RT-PCR?

As the name suggest RAT is rapid test. It gives result in 15-30 minutes.

Disadvantage of RAT over RT-PCR?

Sensitivity of RAT is less than 50 %. So a negative RAT test in symptomatic patients cannot be considered as negative and need to go for RT-PCR to confirm.

What sample from the body is needed to perform these tests?

Swab sample from nose and throat are needed.

When this test is to be done?

As soon as the symptoms appear and/or if exposed to COVID patient.

When the test should get repeated?

If the patient recovered and become symptoms free then there is no need to repeat the test. As the test detect virus not disease. So if patient recovered then no need to worry even if test comes positive.

Only if the employer asks for it to join the duty then a test can be repeated.

Or for travel purpose.

Disclaimer:The author, Dr. Niket Rai, MBBS, MD, Maulana Azad Medical College and Associated Lok Nayak Hospital, Delhi. Views expressed are personal

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first published:June 01, 2021, 09:01 IST