PCR means polymerase chain reaction. It’s a test to detect genetic material from a specific organism, such as a virus. The test detects the presence of a virus if you have the virus at the time of the test. The test could also detect fragments of the virus even after you are no longer infected.
The polymerase chain reaction (PCR) test for COVID-19 is a molecular test that analyzes your upper respiratory specimen, looking for genetic material (ribonucleic acid or RNA) of SARS-CoV-2, the virus that causes COVID-19. Scientists use the PCR technology to amplify small amounts of RNA from specimens into deoxyribonucleic acid (DNA), which is replicated until SARS-CoV-2 is detectable if present. The PCR test has been the gold standard test for diagnosing COVID-19 since authorized for use in February 2020. It’s accurate and reliable.
You should receive your test results as early as 24 hours after sample collection, but sometimes it can take a few days, depending on how long it takes the sample to reach the laboratory.
Because the PCR test is so sensitive, it can detect very small amounts of virus material. This means that the test can continue to detect fragments of SARS-CoV-2 virus even after you’ve recovered from COVID-19 and are no longer contagious. So you may continue to test positive if you’ve had COVID-19 in the distant past, even though you can’t spread the SARS-CoV-2 virus to others.
Prolonged infection in immunocompromised individuals can occur where they shed infectious virus for months. Also, healthy people can become reinfected. If you test positive for SARS-CoV-2 but you think you might have already recovered from COVID-19, please discuss with a healthcare provider.
If you have COVID-19 it can take several days to develop symptoms — but you’re contagious during this time. You are no longer contagious 10 days after your symptoms began.
The best way to avoid spreading COVID-19 to others is to:
Your healthcare provider may recommend testing for COVID-19 if you have any of the following symptoms:
Real time RT–PCR is a nuclear-derived method for detecting the presence of specific genetic material in any pathogen, including a virus. Originally, the method used radioactive isotope markers to detect targeted genetic materials, but subsequent refining has led to the replacement of isotopic labelling with special markers, most frequently fluorescent dyes. This technique allows scientists to see the results almost immediately while the process is still ongoing, whereas conventional RT–PCR only provides results at the end of the process.
Real time RT–PCR is one of the most widely used laboratory methods for detecting the COVID-19 virus. While many countries have used real time RT–PCR for diagnosing other diseases, such as the Ebola virus and Zika virus, many need support in adapting this method for the COVID-19 virus, as well as in increasing their national testing capacities.
A virus is a microscopic package of genetic material surrounded by a molecular envelope. This genetic material can be either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
DNA is a two-strand m
RNA is generally a one-strand molecule that copies, transcribes and transmits parts of the genetic code to proteins so that they can synthetize and carry out functions that keep organisms alive and developing. Different variations of RNA are responsible for copying, transcribing and transmitting.olecule that is found in all organisms, such as animals, plants and viruses, and which holds the genetic code, or blueprint, for how these organisms are made and develop.
Some viruses such as the SARS-CoV-2 coronavirus, which causes COVID-19, only contain RNA, which means that they rely on infiltrating healthy cells to multiply and survive. Once inside the cell, the virus uses its own genetic code — RNA in the case of the COVID-19 virus — to take control of and ‘reprogramme’ the cells, turning them into virus-making factories.
In order for a virus like the COVID-19 virus to be detected early in the body using real time RT–PCR, scientists need to convert the RNA to DNA. This is a process called ‘reverse transcription’. They do this because only DNA can be copied — or amplified — which is a key part of the real time RT–PCR process for detecting viruses.
Scientists amplify a specific part of the transcribed viral DNA hundreds of thousands of times. Amplification is important so that, instead of trying to spot a
ule amount of the virus among millions of strands of genetic information, scientists have a large enough quantity of the target sections of viral DNA
to accurately confirm that the virus is present.
A sample is collected from the parts of the body where the COVID-19 virus gathers, such as a person’s nose or throat. The sample is treated with several chemical solutions that remove substances such as proteins and fats 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 a specific enzyme. Scientists then add additional short fragments of DNA that are complementary to specific parts of the transcribed viral DNA. If the virus is present in a sample, these fragments attach themselves to target sections of the viral DNA. Some of the added genetic fragments are used for building DNA strands during amplification, while the others are used for building the DNA and adding marker labels to the strands, which are then used to detect the virus.
The mixture is then placed in an RT–PCR machine. The machine cycles through temperatures that heat and cool the mixture to trigger specific chemical reactions that create new, identical copies of the target sections of viral DNA. The cycle is repeated over and over to continue copying the target sections of viral DNA. Each cycle doubles the previous number: two copies become four, four copies become eight, and so on. A standard real time RT–PCR set-up usually goes through 35 cycles, which means that, by the end of the process, around 35 billion new copies of the sections of viral DNA are created from each strand of the virus present in the sample.
As new copies of the viral DNA sections 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. Scientists also monitor how many cycles it takes to reach this level in order to estimate the severity of the infection: the fewer the cycles, the more severe the viral infection is.
The real time RT–PCR technique is highly sensitive and specific and can deliver a reliable diagnosis in as little as three hours, though laboratories take on average between six and eight hours. Compared to other available virus isolation methods, real time RT–PCR is significantly faster and has a lower potential for contamination or errors, as the entire process can be carried out within a closed tube. It continues to be the most accurate method available for the detection of the COVID-19 virus.
However, real time RT–PCR cannot be used to detect past infections, which is important for understanding the development and spread of the virus, as viruses are only present in the body for a specific window of time. Other methods are necessary to detect, track and study past infections, particularly those which may have developed and spread without symptoms.
RT–PCR is a variation of PCR, or polymerase chain reaction. The two techniques use the same process except that RT–PCR has an added step of reverse transcription of RNA to DNA, or RT, to allow for amplification. This means PCR is used for pathogens, such as viruses and bacteria, that already contain DNA for amplification, while RT–PCR is used for those containing RNA that needs to be transcribed to DNA for amplification. Both techniques can be performed in ‘real time’, which means results are visible almost immediately, while when used ‘conventionally’, results are only visible at the end of the reaction.
PCR is one of the most widely used diagnostic tests for detecting pathogens, including viruses, that cause diseases such as Ebola, African swine fever and foot-and-mouth disease. Since the COVID-19 virus only contains RNA, real time or conventional RT–PCR is used to detect it.
PCR tests work by:
Certain viruses, including COVID-19, are made up of RNA rather than DNA. For these viruses, the RNA must be changed into DNA before copying. This process is called reverse transcription PCR (rtPCR).
There are different ways to get a sample for a PCR test. Common methods include blood tests and nasal swabs.
During a blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected in a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
A nasal swab may be taken from the front part of your nostrils (anterior nares). It also may be taken from the back of your nostrils, in a procedure known as a nasal mid-turbinate (NMT) swab, or from the nasopharynx, the uppermost part of your nose and throat. In some cases, a health care provider will ask you to do an anterior nares test or an NMT swab yourself.
During an anterior nares test, you will start by tilting your head back. Then you or the provider will:
During an NMT swab, you will start by tilting your head back. Then you or your provider will:
During a nasopharyngeal swab:
There are two types of tests for COVID-19: the PCR test and the antigen test.
The antigen test is typically faster but is less sensitive than the PCR test. Because the antigen test is not as accurate as PCR, if an antigen test is negative, your healthcare provider could request a PCR test to confirm the negative antigen test result.
The vaccines significantly reduce — but don’t eliminate — your risk of infection. The risk of a severe illness or death from a breakthrough infection is very Low.
If you have symptoms of COVID-19 or were exposed to people who have symptoms or have tested positive, you may want a test. First, talk with your healthcare provider. They will review your symptoms in person or on a video appointment. If needed, the provider orders a test and helps you find a testing location and time. Keep in mind that if you’ve been exposed to the SARS-CoV-2 virus but don’t have symptoms, call the testing site first to make sure they can accommodate you.
You can also call or check the websites of your local hospitals in your health insurance network or check with community health centers or urgent care centers. The U.S. Department of Health and Human Services provides links to find community-based testing sites in your state. You can also check your state or local health department websites for the latest information on testing locations. The Centers for Disease Control provides links to these state and local health departments.
You don’t need any special preparations for a PCR test.
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
A nasal swab may tickle your throat or cause you to cough. A nasopharyngeal swab may be uncomfortable and cause coughing or gagging. All these effects are temporary.
PCR tests are an accurate and reliable method for identifying many infectious diseases. And because they are often able to make diagnoses before symptoms of infection occur, PCR tests play a crucial role in preventing the spread of diseases.
There is two kinds of covid results Positive and Negative details are following.
Prevention & Advice