Whether you watch the news, read the paper or surf on social media, the chances are you will have come across the PCR-test method to determine whether someone is contaminated by the COVID-19 virus or not.
But whilst journalists are quick to explain how the test is carried out (nasal swabs, blood test etc.) the science behind it is rarely explained. PCR is currently the front line laboratory assay for the screening and diagnosis of COVID-19, in conjunctions with the clinical symptoms and imaging (CT scan).
Serological testing or antibody tests can be used to identify whether people have been exposed to the virus.
1. Molecular Testing: PCR
PCR stands for Polymerase Chain Reaction and simply said means “molecular photocopying”. Basically when a PCR test is performed, the cells of the patient are screened and if the RNA virus is present then it is massively photocopied such that a significant quantity of the virus can be seen on the imaging apparatus. If no virus is present, then the photocopying is blank and the test comes back negative. For this photocopying to be successful however, the “photocopying machine” needs to be able to detect the virus’ RNA present in the cells…and that is why the efficiency of the PCR testing is dependent on the method used.
Data has shown that PCR-tests based on human cells present in human sputum (thick mucus) are 72% accurate whereas those performed on cells derived from nasal swabs are only 63% accurate, meaning that in 37% of cases, patients are coronavirus-positive and yet appear as negative.
Similarly PCR tests performed on cells derived from a bronchoalveolar lavage fluid are 93% reliable whereas tests derived from a fibro bronchoscope brush biopsy, pharyngeal swabs, feces, blood and urine are respectively reliable only up to 46%, 32%, 29%, 1% and 0%. This can be explained by the virus’ lifecycle and therefore the viral load present in the cells at the time when patients get tested.
Most frequently, patients are tested when the symptoms they exhibit could indicate a variety of different diagnoses and therefore a test is performed to confirm or infirm that they are coronavirus positive.
At this stage in the life cycle of the virus, it is little present in the pharynx, the feces, the blood or the urine. Similarly, whilst measuring the temperature of people upon entry in a country is an efficient method to screen for patients who are already sick, many patients will not exhibit fever at all and others will be asymptomatic carriers.
2. Serological Testing
The additional question for the medical community and the public, is the level of immunization of an individual, meaning that he faced to the disease by activating his immune system and produced antibodies.
This could mean that he had the infection or later on, when will be available, that he immunized after vaccination. They enable to measure who has been infected and who is potentially immune to the virus. The production of antibodies production (IgM, IgM and IgG) starts within a week after the beginning of the disease and IgG will remain positive several months after disease.
The assays for measuring the presence of antibodies are called serology tests. Serology tests are performed on blood samples using different kind of assays, lateral flow assays (rapid tests but not quantitative), semi-automated immunoassays (ELISA) or fully automated quantitative immunoassays with chemiluminescence detection (CLIA).
Testing will play a crucial role in stopping the spread of Covid-19 and needs to be deployed across the global healthcare ecosystem as soon as possible. Singapore, South Korea and Germany have demonstrated a higher testing capability and consequently a better management of the pandemic. Only once we have a clear idea of who is vulnerable and who isn’t, can we effectively start stopping the spread of the virus.
Now no matter what, stay safe, be prepared. and stay connected for more articles from Absolutys Institute.
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