A lot of commotion in the news lately concerning tests. The good news is that newer and faster
tests are being developed with a saliva gargle test
on the verge of approval in BC.
At CNN though, there was a headline in
the ticker tape that turned heads and spread
like a virus (pardon the pun) in all sorts of
conspiracy web pages. “Health experts warn
Covid-19 tests aren’t just slow, they’re too
sensitive”. NSN reached out to Dr. Christos
Karatzios Assistant Professor of Paediatrics at
the Montréal Children’s Hospital for a muchneeded clarification.
The doctor started by saying that people
didn’t understand what they were reading and
of course they told him that positive tests are
His answers are all here:
Yes, the tests are sensitive. They HAVE to be.
They pick up genetic RNA material of SARS-2-
CoV in your nose.
No, the tests do not pick up false positive
results. The nasal PCR test for SARS-2-CoV
ONLY DETECTS SARS-2-CoV and nothing
else. So, it doesn’t pick up “other coronaviruses”.
The false positive rate is extremely low and
mostly has to do with lab error (i.e. contamination
with a positive sample in the lab). This is very rare.
The false negative rate is much higher due to
various factors (i.e. sampling error – not getting
the swab deep enough in the nasal passage, or
sample timing – too early in the disease or too
The test is a “YES” or “NO” test for the presence of the virus. It tells you nothing about live
(growable and infectious virus), or dead remnant
pieces of the RNA (therefore not infectious virus).
Someone can remain positive for weeks as many
of my friends reading this have. Some have stayed
positive for 6 weeks. It does NOT mean they are
contagious all these weeks.
Immune suppressed people and people with
severe COVID-19 (in ICU on a ventilator etc),
may be contagious for 3-4 weeks.
For the rest of the healthy population (i.e. children in school) who got COVID-19, live infectious virus stops shedding after about 10 days.
Your test can still pick up genetic pieces after
this but the virus is dead.
A quick hint: do you have fever and a bad cough
and diarrhea? You’re contagious until all gone.
You were contagious 1-2 days before it all started.
A PCR is a test that checks for the genetic
material and, if present, enzymes in the test start
making copies of the genetic material until we
are able to detect it. The checking happens in
cycles. The less cycling needed to detect, the
more genetic material that exists in that swab
and so the Ct (cycle threshold) is LOW. The
less genetic material that exists in the swab, the
higher is the Ct.
This is NOT a viral load. It does not give you
a number of copies of the virus per mL of snot
or per nasal surface cells in your nose. This is
the major mistake many papers and scientists
A viral load has a curve – so therefore it rises
- and it falls -. Scientists can only presume that
with the current PCR test the Ct rises and falls
too. But a viral load curve needs a denominator
to be constant and that is the sample. In HIV
we know the viral load of a patient because we
sample a constant blood volume. For a respiratory virus like SARS-2-CoV, it depends on the
sample and this isn’t constant as many of you
who have had the test done know – you thrash
about because it’s uncomfortable and there may
be less cells in the nasal swab. Or the testing
person samples the front of the nose where the
virus doesn’t live well when compared to the back
of the nose. There is no constant denominator
unlike a blood test – and we don’t have those
developed for this pandemic.
Unlike flu (300 viral particles in droplets but
3 if airborne), Shigella causing shigellosis and
dysentery (1-10 bacteria), and Salmonella causing typhoid fever or other salmonellosis diarrheal
illnesses (100-200 bacteria) we do NOT know
the infective dose of the SARS-2-CoV.
So, even if we knew the viral load of the test
we would not know what it meant. Does it take
1, 10, 10,000 viral particles to cause COVID19? What about black people who have higher
numbers of ACE receptors for the virus, or kids
and mild asthmatics who have less? How many
viruses are needed to cause disease in them?
We just don’t know and we will not know for
a while. Remember we are learning as we live.
We are building the boat as it is sailing during
Also, what if the viral load is low…can a child
return to school let’s say as was suggested by
CNN? How do you know if the viral load is on
the upswing (early in the disease and the patient
is asymptomatic and will have a big viral load
in a few hours when he/she becomes presymptomatic) or late in the disease as it has passed?
In conclusion, yes, the tests are sensitive but
they are specific for this virus only. If positive
we don’t know where you are on the disease
timeline unless you have symptoms. Isolation
and contact tracing are the only way to control
this disease apart from wearing masks, washing
hands, and keeping distances. Unfortunately, we
can’t predict who can go to school or who can’t.
If an outbreak happens based on positive tests,
a school may need to fully close. Unfortunately,
that’s the limits of our technology. Maybe in a
few months we will know more.