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When is a Duck not a Duck? The Pitfalls of COVID-19 Testing

So, you had a cough and cold and you were worried about COVID….but you went and got a rapid test and its negative so all is good right? Wrong! 

No test is perfect and this is to help you understand some pitfalls of Covid testing.  

Let’s start with the rapid “blood” test. This is looking for antibodies to the virus, and not directly for the virus itself.  

When we get the virus for the first time, it may take a while for our immune system to mount a response and produce antibodies.  

If the test is done too soon, for example before symptoms start, or even in the first 2 weeks after getting it when you may even have symptoms, it may read negative when you are in facet positive, a so called “False negative”! Then you might be falsely reassured and unwittingly go round infecting others.  

 

So, an antibody test MAY be useful after your symptoms have passed, and if you never managed to get the proper “throat swab” PCR test and you are wondering if in fact it was COVID you had! 

 

What if you wait, do the antibody test more than two weeks after your symptoms started and you have antibodies Does that mean you are now “immune” to COVID? 

 

Er, we are still not so sure. It seems that antibody levels wane after 3 months or so, so we are not sure how long that immunity may last…. this is also complicated by the fact that there are different strains of COVID and it may be that the virus mutates very so often, just like the Flu, so your immunity may be short lived! 

 

On top of this, some people can fight of the virus with a different part of the immune system that doesn’t use antibodies. So you might actually have fought off the virus successfullyand may even have immunity that we can’t measure on the test, but then have no antibodies.  

 

Confusing right?  

 

What about the “swab” or fancy RT-PCR  test? 

 

The swab tests for actual virus particles in the nose/throat.  It is not 100% either!  No test is I am afraid.  The swab is most accurate if done days 3-5 of symptoms.  If done in asymptomatic people right after exposure, it may be too early to get a positive result. Similarly, if taken very late in the illness like day 10, the chance of picking up the virus starts to fall.  

 

It is very specific and moderately sensitive. So, what does that mean? Well, it means that if you get a positive result it is likely to be a true positive, but a negative result is less reliable. So, we tend to believe the positive results, but if a person has a negative test, that doesn’t automatically mean they are truly negative and that they don’t have COVID 19 and are not infectious. 

 

That’s why in very suspicious cases you will see us asking for 2 negative tests. ..That’s because if it looks like a duck, and quacks like a duck and flies like a duck it is probably a duck…especially if it comes from an area where there is known to be a flock of ducks living. (Like say, you shot it in the middle of a swamp) So, if someone plucks a feather from that duck and analyses it and tells you it is not a duck then would you believe them?   

 

Similarly, if a person has been exposed to someone with COVID 19, and they then develop the classic fever cough, anosmia etc, OR we know that we are in the middle of a Pandemic or local spike and someone gets a cough and cold, we have a high suspicion of COVID and just one negative test is not going to reassure us.  

 

To complicate things, we will shortly be going into the “Flu Season” and it’s hard to tell the difference clinically between the Influenza virus and COVID 19! But that’s for a whole other discussion.  

 

In short, please don’t just take yourself off for a test without consulting a health professional who is up to date with testing and timing and symptoms. The test may not mean what you think it does, and you may even need a completely different test!