PCR Test

A polymerase chain reaction (PCR) test is performed to detect genetic material from a specific organism, such as a virus. The test detects the presence of a virus if you are infected at the time of the test. The test could also detect fragments of virus even after you are no longer infected.[1]

Background

PCR tests are used to directly detect the presence of an antigen, rather than the presence of the body’s immune response, or antibodies. By detecting viral RNA, which will be present in the body before antibodies form or symptoms of the disease are present, the tests can tell whether or not someone has the virus very early on.

By scaling PCR testing to screen vast swathes of nasopharyngeal swab samples from within a population, public health officials can get a clearer picture of the spread of a disease like Covid-19 within a population.

It’s worth noting that PCR tests can be very labour intensive, with several stages at which errors may occur between sampling and analysis. False negatives can occur up to 30% of the time with different PCR tests, meaning they’re more useful for confirming the presence of an infection than giving a patient the all-clear.[2]

COVID-19 and PCR Testing

A PCR test for COVID-19 is a test used to diagnosis people who are currently infected with SARS-CoV-2, which is the coronavirus that causes COVID-19. The PCR test is the “gold standard” test for diagnosing COVID-19 because it’s the most accurate and reliable test.

The nose swab PCR test for COVID-19 is the most accurate and reliable test for diagnosing COVID-19. A positive test means you likely have COVID-19. A negative test means you probably did not have COVID-19 at the time of the test. Get tested if you have symptoms of COVID-19 or have been exposed to someone who tested positive for COVID-19.

Warwick Medical School honorary clinical lecturer Dr James Gill said: “During the course of the outbreak, the PCR testing has been refined from the initial testing procedures and with the addition of greater automation to reduce errors. As such, we now have an 80-85% specificity – i.e. the chance the test is detecting the virus.

“Remember as we are looking at swabs taken from people, who have lots of other organisms floating around, we are essentially dealing with the question of how ‘right’ the result we are looking at is.”

How a PCR Test is Done

Also called a molecular test, this (Coronavirus) COVID-19 test detects genetic material of the virus using a lab technique called polymerase chain reaction (PCR). 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. Results may be available in minutes if analyzed onsite or a few days — or longer in locations with test processing delays — if sent to an outside lab. PCR tests are very accurate when properly performed by a health care professional, but the rapid test can miss some cases.[3]

How long does it take to get the PCR Test results

PCR tests detect the presence of the virus's genetic material using a technique called reverse transcriptase polymerase chain reaction, or RT-PCR. For this test, a sample may be collected through a nasal or throat swab, or a saliva sample may be used. The sample is typically sent to a laboratory where Coronavirus RNA (if present) is extracted from the sample and converted into DNA. The DNA is then amplified, meaning that many of copies of the viral DNA are made, in order to produce a measurable result. The accuracy of any diagnostic test depends on many factors, including whether the sample was collected properly, when during the course of illness the testing was done, and whether the sample was maintained in appropriate conditions while it was shipped to the laboratory. Generally speaking, PCR tests are highly accurate. However, it can take days to over a week to get the results of a PCR test'.

Antigen tests detect specific proteins on the surface of the coronavirus. They are sometimes referred to as rapid diagnostic tests because it can take less than an hour to get the test results. Positive antigen test results are highly specific, meaning that if you test positive you are very likely to be infected. However, there is a higher chance of false negatives with antigen tests, which means that a negative result cannot definitively rule out an active infection. If you have a negative result on an antigen test, your doctor may order a PCR test to confirm the result.[4]

PCR Testing in Zimbabwe

As Zimbabwe joined the rest of the world in fighting the Covid-19 pandemic, most Zimbabweans questioned if the country was able to test as many cases as possible. Others were questioning if there even was such a machine to do the testing. National Microbiology Reference Laboratory (NMRL) Acting Coordinator, Mr Xavier Mazarura indicated that indeed the country had a testing centre and explained, in detail, about the testing process, machinery and personnel requirements.

“Zimbabwe currently has one COVID-19 testing centre situated at the National Microbiology Reference Laboratory (NMRL) situated at Sally Mugabe Central Hospital (formerly Harare Central Hospital) in Harare therefore the laboratory receives samples from all provinces for testing.

The NMRL can test up to 400 samples per day using the real-time Polymerase Chain Reaction (PCR) machine. The tests are conducted using the PCR diagnostic method which has a 100% accuracy rate. The testing process takes about 5 hours,” said Mazarura. For total quality management purposes, tested samples are also send to South Africa for further confirmation of the results.

“In the event that the NMRL testing centre becomes overwhelmed, other samples can be send to the Biomedical Research and Training Institute (BRTI), National Virology Reference Laboratory at Parirenyatwa Hospital and the African Institute of Biomedical Science and Technology (AiBST) at Wilkins Hospital,” revealed Mazarura. Some people were worried to the extent of questioning if the country had the required manpower and enough machines to do the job expeditiously. However, Mazarura allayed those fears pointing out the available machines and workforce.[5]




References

  1. [1], Cleveland Clinic, Accessed: 16 January, 2021
  2. Chloe Kent, [2], Medical Device Network, Published: 3 April, 2020, Accessed: 16 January, 2021
  3. [3], Mayo Clinic, Accessed: 16 January, 2021
  4. [4], Harvard Health Publishing - Harvard Medical School, Published: March, 2020, Accessed: 16 January, 2021
  5. [5], Ministry of Health and Child Care, Published: 10 April, 2020, Accessed: 16 January, 2021