There are two different types of tests for detection of covid-19 – diagnostic tests and antibody tests.
- A diagnostic test can show if you have an active coronavirus infection and should take steps to quarantine or isolate yourself from others. Currently there are two types of diagnostic tests– molecular tests, such as RT-PCR tests, that detect the virus’s genetic material, and antigen tests that detect specific proteins from the virus.
- An antibody test looks for antibodies that are made by your immune system in response to a threat, such as a specific virus. Antibodies can help fight infections. Antibodies can take several days or weeks to develop after you have an infection and may stay in your blood for several weeks or more after recovery. Because of this, antibody tests should not be used to diagnose COVID-19. At this time researchers do not know if the presence of antibodies means that you are immune to COVID-19 in the future.
RT-PCR test:
The COVID-19 RT-PCR test is a real-time reverse transcription polymerase chain reaction (rRT-PCR) test for the qualitative detection of nucleic acid from SARS-CoV-2. A fluid sample is collected by inserting a long nasal swab (nasopharyngeal swab) into your nostril and taking fluid from the back of your nose or by using a shorter nasal swab (mid-turbinate swab) to get a sample. In some cases, a long swab is inserted into the back of your throat (oropharyngeal swab), or you may spit into a tube to produce a saliva sample. PCR tests are very accurate when properly performed by a health care professional.
In this assay the sample is treated with several chemical solutions that remove substances such as proteins and lipids and that extract only the RNA present in the sample. This extracted RNA is a mix of the person’s own genetic material and, if present, the virus’ RNA.
The RNA is reverse transcribed to DNA using reverse transcriptase enzyme. Then primers that are complementary to specific parts of the transcribed viral DNA added. If the virus is present in a sample, primers attach themselves to target region of the viral DNA. Some of the primers are labeled. The mixture is then placed in thermo cycler machine. The cycle is repeated over and over to continue copying the target regions of viral DNA. A standard real time RT–PCR set-up usually goes through 35 cycles.
As new copies of the viral DNA regions are built, the marker labels attach to the DNA strands and then release a fluorescent dye, which is measured by the machine’s computer and presented in real time on the screen. The computer tracks the amount of fluorescence in the sample after each cycle. When a certain level of fluorescence is surpassed, this confirms that the virus is present. In order to estimate the severity of the infection number of cycles that is taken to reach specific level is also monitor: the fewer the cycles, the more severe the viral infection is.
Evidence suggests that testing tends to be less accurate within three days of exposure, and the best time to get tested is five to seven days after you were exposed. Tests are even more accurate when patients are exhibiting symptoms. Testing too close to an exposure could result in samples that don’t contain enough of the virus’ genetic material to register as a positive. A negative PCR test does not mean that an individual is free of infection, but rather only that, at that particular moment, the sample did not contain viral levels at a high enough concentration to be measured as a positive.
