On Wednesday (9 Dec) a Royal Caribbean ship on a cruise to nowhere had to turn back ahead of schedule after an 83-year-old man on board tested positive for Covid-19.
All his close contacts were isolated, and later tested negative.
However, when the man’s original sample was sent to the National Public Health Laboratory (NPHL) for a follow-up test, he tested negative.Â
A subsequent test of a second fresh sample by NPHL also came back negative.
NPHL will conduct another test tomorrow to confirm his Covid-19 status, but this has certainly left many scratching their heads.
How could he test positive one day and negative the next? Was it a false positive?
Well, before we get into that, we first have to determine what exactly a false positive is.
Specificity & SensitivityÂ
According to MIT Medical, when we talk about the accuracy of a laboratory test, we’re actually referring to two different types of accuracy – sensitivity and specificity.
Sensitivity is a test’s ability to determine when an individual is infected.
Conversely, specificity is its ability to tell us when an individual is not infected.
“A test that is very sensitive is less likely to give false-negative results, and a test that is highly specific is less likely to give false positives”, it said.
One Covid-19 test is the polymerase chain reaction (PCR) test, which is a molecular test, meaning it works by detecting genetic material from the coronavirus.
When someone is infected, this genetic material can be found in the nose and upper throat, which are the areas swabbed for tests.
PCR tests are highly specific, meaning it almost never gives a false positive.
On the other hand, the test is not equally sensitive.
This means that if the specimen collection is not done perfectly, or if a patient is in an early stage of infection or already partially recovered from the illness, their sample might not contain enough viral material to come back positive.
In other words, false negatives are possible with Covid-19 PCR tests.
The only problem is that just like face masks, not all Covid-19 tests are created equal.
Antigen Rapid Test vs PCR Test
At the moment, there are two types of diagnostic tests for Covid-19: antigen rapid tests and PCR tests.
Unlike PCR tests, antigen rapid tests look for proteins on the surface of the virus called antigens.
Antigen tests are known for their speed, as they can deliver results within 30 minutes.
They are also cheaper and easier to administer than PCR tests.
However, PCR tests remain the gold standard for testing as they are the most accurate tests available today.
According to The Straits Times, antigen rapid tests have lower sensitivity and specificity, meaning they carry a higher risk of false positives and false negatives.
This is why the U.S. Food and Drug Administration warned clinical laboratory staff and healthcare providers in November that false positives can occur with Covid-19 antigen rapid tests.
The Bottom Line
So, while false positives are certainly possible with Covid-19 tests, they typically occur with antigen rapid tests.
The 83-year-old man who tested positive on board Royal Caribbean’s cruise ship Quantum of the Seas had undergone a PCR test, meaning it’s unlikely that his test on board was a false positive.
However, the man had also undergone a mandatory PCR test, just as every other passenger, prior to boarding and tested negative.
It’s possible that his very first test had returned a false-negative result, but it’s hard to say.
If the man did indeed have the virus in his system and tests negative tomorrow for the third time in a row, it could mean that he’s partially recovered from the illness.
However, we’ll have to wait for an explanation from the experts before jumping to any conclusions.
Featured Image: aslysun / Shutterstock.com
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