Different types of Tests for Covid-19 available in India
In the bid to scale up testing for COVID-19 or the infection caused by the Coronavirus, the Government of India apex body ICMR has introduced various types of tests for the detection of the virus/ infection. Understand the differences, which test to go for, and under what circumstances.
RT PCR Test
Real-time Reverse Transcription Polymerase Chain Reaction (rRT-PCR) Test
This is the most commonly used test to detect Covid-19 patients. A swab taken from nose or throat of a patient, sputum, saliva or deep airway material collected via suction catheter act as samples. This sample contains RNA [Ribonucleic acid] of corona virus but amount of RNA in the sample is very small to be detected by the test. Hence single stranded RNA is first converted into a double stranded DNA, the technique is called Reverse Transcription using an enzyme, Reverse Transcriptase and then amplifying DNA by Polymerase Chain Reaction technique so that the genetic material of the virus becomes sufficient enough to be detected by the test.
The part of the genetic material of Covid-19 virus that mutates least is chosen for amplification. Small pieces of DNA meant to bind to the selected DNA sequence of Covid-19 virus, known as Primer, is used along with a fluorescent dye, which acts as a probe in the testing technique.
Sample of patient, probe and primer are left in the PCR machine for the binding process to happen. Presence of virus is marked by a fluorescent signal.
The time taken for the testing process is four to eight hours but it may take about 24 hours for the result to be ready because the sample also needs to be transported to the testing lab. This is the costliest test available for Covid-19 testing.
In the first week throat swab sample gives good results but in the second week deep airway material or sputum are good samples as the virus replicates in the lungs during second week. Once the patient becomes symptomatic, which occurs in the beginning of the second week, the viral load in the upper respiratory tract becomes lesser and lesser and hence samples should be taken from deep airway [sputum] rather than throat swab.
Nasal swab is taken as a sample and immersed in a solution which deactivates the virus. This solution is now put on a strip which is already impregnated with artificial antibodies against covid-19 virus. If the sample of patient contains antigens of Covid-19 virus, they will bind with the artificial antibodies present in the test strip and lines appear. The result is ready within 30 minutes. Since antigen detection does not require any type of amplification process hence time taken is very less but as the sample may not contain sufficient amount of viral load, the test may be False Negative. It does not require expensive machines or extensive training.
If the test is positive no other confirmation is needed. However negative patients need to be confirmed by RT PCR testing. Advantage of this test is that it gives result in less than 15 minutes and thus lowers the load for the RT PCR testing which takes much longer time.
Antibody Test [Serological Tests]
Blood of patient is taken who have recovered from Covid-19 infection and contain antibodies against the virus. These antibodies may be IgG or IgM. IgG antibodies appear 10–14 days after infection and peak around 28 days after infection. Because antibodies appear late in the blood, hence their presence cannot be used as acute infection marker. However, their presence confirms that the patient has recovered from Covid-19 virus. These antibodies are used as Plasma therapy for seriously ill patients having Covid-19 infection.
Antibodies can be detected in the blood using ELISA technique [Enzyme Linked Immunosorbent Assay]. A plate is coated with a viral antigen, which is a protein against the viral spikes. Samples are incubated with the antigen. If blood sample of patient contains antibodies against the viral spikes, they will bind with the antigen and the antigen antibody complexes are detected in form of colored lines as appear in Home pregnancy test.
Antibody tests are useful to know whether the population has been exposed to the infective agent. It can screen a large population in a very short span of time.
The samples used in this test are taken from nose or throat and dipped in a solution which deactivates the virus. The solution is then placed on a cartridge. This cartridge is inserted into a machine which extracts genetic material from the sample. This is followed by RT PCR. This purified genetic material is now added into a tube which contains RT PCR reagents. This solution is now applied to a microchip which is then put into another machine where reverse transcription and PCR takes place. Sample preparation in this test is automated hence results are available within half an hour. As this test is very quick and portable, it can be carried out in containment zones.
This is an indigenously developed test based on CB-NAAT Cartridge Based Nucleic Acid Amplification Test. This test was used to detect Tuberculosis.
Written by Dr Ashwani Ghai