If more tests are done, more people will get a false positive result. This means that the test is positive, while you are not infected with the corona virus at all.
The number of false positive results can even exceed the number of real positive results. That sounds illogical. It’s a complicated story, but here’s the thing:
The corona test has a margin of error. There is a chance that the test is positive even though someone is not infected. That chance is about 1 percent with the corona test.
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Suppose a group of 10,000 people is tested, and that 1 percent of those people is actually infected. Then there are 100 infections.
But 1 percent of people who are not infected also get a positive result due to the margin of error. That is another 99 people, who are wrongly quarantined.
Not all infections are detected
There is another factor at play: the test also has a sensitivity. That is to say: what is the chance that an infection will actually be detected in the test? That chance depends on several factors, such as how quickly you get tested and how sick you are. Ideally, the sensitivity of a corona test is about 85 percent.
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The test is therefore negative for 15 percent of the infections, even though someone is infected. Of the 100 infections, 85 are then detected.
In summary: of the 100 people who are infected, 85 get a positive result. But of the people who are not infected, 99 get a positive result. Most positive results, almost 54 percent, are therefore incorrect.
More than a quarter wrong
The example exaggerates reality a bit. In the calculation we have assumed that 1 percent of the tested persons is actually infected. That percentage is actually higher: last week it was almost 3 percent.
We can also fill in the same sum with that 3 percent. The number of infections found is therefore higher, because more infected people are being tested. The number of false positives is not increasing, so proportionally there are more accurate test results. But even with an infection rate of 3 percent, more than a quarter of the positive results are incorrect.
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Little can be done about it
The reliability therefore depends on the degree of contamination at the test line. The more people get tested while they are not infected, the greater the chance of a false positive result.
Unfortunately, little can be done about incorrect test results, says RIVM virologist Chantal Reusken against NRC. “The most important thing is people’s behavior: if you have a cold, go to the test street as soon as possible, and don’t go if you don’t have any symptoms.”
Reusken says about the people who have to be wrongfully quarantined: “You do that for society. You take one for the team.”
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