How do you know if you have a virus? - Cella Wright

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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? Cella Wright explores the science of PCR tests and immunoassays.

Lesson by Cella Wright, directed by Hype CG.

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.