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  • A new virus emerges and spreads like wildfire.

  • In order to contain it, researchers must first collect data about who's been infected.

  • Two main viral testing techniques are critical: one tells you if you have the virus and the other shows if you've already had it.

  • So, how exactly do these tests work?

  • PCR, or polymerase chain reaction testing, targets the virus's genetic material in the body and is used to diagnose someone who is currently infected.

  • Yet, this genetic material may be present in such imperceptible amounts that actually detecting it is difficult.

  • This is where PCR comes in: it's widely used to amplify genetic information to large enough quantities that it can be readily observed.

  • To develop a PCR test for a never-before-seen virus, researchers first sequence its genetic material, or genome, and identify regions that are unique to that specific virus.

  • PCR then targets these particular segments.

  • A PCR test begins by collecting a sample: this can be blood for hepatitis viruses, feces for poliovirus, and samples from the nose or throat for coronaviruses.

  • The sample is taken to a central laboratory where PCR is performed to test for the presence of the virus' genome.

  • Genetic information can be encoded via DNA or RNA.

  • HPV, for example, uses DNA, while SARS-CoV-2, the cause of COVID-19, uses RNA.

  • Before running the PCR, the viral RNA, if present, must be reverse transcribed to make a strand of complementary DNA.

  • Researchers then run the PCR.

  • If the virus is present in the sample, its unique regions of genetic code will be identified by complementary primers and copied by enzymes.

  • One strand of DNA becomes hundreds of millions, which are detected using probes marked with fluorescent dye.

  • If the PCR machine senses fluorescence, the sample has tested positive for the virus, meaning the individual is infected.

  • Immunoassays, on the other hand, tap into the immune system's memory of the virus, showing if someone has previously been infected.

  • They work by targeting virus-specific antibodies generated by the immune system during infection.

  • These are specialized classes of proteins that identify and fight foreign substances, like viruses.

  • Immunoassays may detect IgG antibodies, the most abundant class, and IgM antibodies, the type that's first produced in response to a new infection.

  • The presence of IgM antibodies suggests a recent infection, but since it can take the body over a week to produce a detectable amount, they're unreliable in diagnosing current infections.

  • Meanwhile, IgG antibodies circulate for an extended period after infection; their presence usually indicates that someone was exposed and recovered.

  • Before the immunoassay, health professionals draw blood from an individual.

  • This sample then comes into contact with a portion of the virus of interest.

  • If the body has, in fact, been exposed to the virus in the past, the body's virus-specific antibodies will bind to it during the test.

  • This reaction produces a change in color, indicating that the sample tested positive and that the individual has been exposed to the virus.

  • Immunoassays are especially important when it comes to retroactively diagnosing people who were infected but went untested.

  • And there's exciting potential for those who have developed immunity to a virus: in some cases, their blood plasma could be used as treatment in people who are currently fighting it.

  • PCR and immunoassays are always in the process of becoming more accurate and efficient.

  • For example, innovations in PCR have led to the use of self-contained testing devices that relay results within one hour.

  • Digital PCR, which quantifies individual pieces of target DNA, shows promise in further boosting accuracy.

  • And although immunoassays are difficult to develop quickly, researchers in Singapore were able to create one for SARS-CoV-2 even before COVID-19 was declared a pandemic.

  • These tests, along with the scientists who develop them and the health professionals who administer them, are absolutely essential.

  • And when deployed early, they can save millions of lives.

  • We know that our bodies produce virus-specific antibodies, but how?

  • Learn more about your bodies defenses with this video, or find out how a very different medical test works with this one.

A new virus emerges and spreads like wildfire.

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B2 US TED-Ed pcr genetic sample infected dna

How Do Virus Tests Actually Work? - Cella Wright

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    Celine Chien posted on 2020/08/08
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