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 much-needed 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 “false positives”.
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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 late).
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 are making.
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 COVID-19? 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 this pandemic.
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.