Does ‘rapid’ mean ‘less accurate’ for coronavirus tests?

The word "rapid" defines any method which is significantly faster than the ones we use today to detect SARS-Cov2. Rapid tests take less than an hour or even a few minutes. They are often performed at Point of Care (POC), meaning that the samples are not taken to a laboratory but instead are processed on-site. This is faster and cheaper.

Published on

10 Sep 2020

The word "rapid" defines any method which is significantly faster than the ones we use today to detect SARS-Cov2. Rapid tests take less than an hour or even a few minutes. They are often performed at Point of Care (POC), meaning that the samples are not taken to a laboratory but instead are processed on-site. This is faster and cheaper.

Myth: Rapid coronavirus tests are less accurate.

Fact: Rapid tests can be as accurate as standard ones. Accuracy does not necessarily depend on speed.

What is a “rapid” test for the coronavirus? 

The word “rapid” defines any method which is significantly faster than the ones we use today to detect SARS-Cov2 (the coronavirus that causes COVID-19).
The golden standard test for SARS-COV2 is based on a technology called RT-PCR. The sample must be sent to a qualified laboratory and takes several hours to give a result.
Rapid tests take less than an hour or even a few minutes. They are often performed at Point of Care (POC), meaning that the samples are not taken to a laboratory but instead are processed on-site. This is faster and cheaper.
Faster and more widespread tests will be key to controlling the COVID-19 pandemic and limiting lockdowns. With rapid tests, more people can be checked at work or during travel. Carriers can be detected earlier and quarantines can be shortened for those who test negative.

What is NOT a “rapid” test for the coronavirus?

To avoid confusion, we must distinguish between direct or molecular tests and serological tests.
Molecular tests look directly at the virus and detect its presence in the organism, whereas serological tests can tell whether a person has developed the antibodies against the virus – and therefore whether they have been infected sometime in the past, even years before.
Serological tests for SARS-Cov-2 are typically done on a blood prick while molecular tests are done on a nose-and-throat swab or a sample of sputum.
Serological tests are often referred to as “rapid tests” because they can be performed in a few minutes, but they are not designed to detect the virus. Therefore, they are not rapid tests for SARS-Cov2.
Only molecular tests can tell whether a person carries the virus. Therefore,  “rapid test” refers to molecular, not serological procedures.

Myth: Serological tests are the rapid tests for the coronavirus

Fact: Serological tests are not designed to detect the virus. Therefore, they are not rapid tests for SARS-Cov2.

What makes for an accurate test?

The accuracy of a diagnostic test is defined by two statistical measures: sensitivity and specificity.
Sensitivity is the ability to detect the virus in carriers. A 100 per cent sensitivity means that a test will identify all the individuals carrying the virus, while a 98 per cent sensitivity means that the test will miss 2 per cent of carriers, and so on. Carriers who are missed by the test are defined as false negatives.
Specificity is the ability to correctly exclude the individuals who are not carriers. In a test that is 100 per cent specific, no one without the virus will test positive. If the specificity is 98 per cent 2 out of 100 of those who tested positive are not carriers and therefore are called false positives.
A perfect test would have a 100 per cent sensitivity and specificity, of course. However, this is rarely the case in real applications.
Most tests have a degree of error, and therefore their accuracy is not perfect. And there is typically a trade-off where you can’t improve sensitivity without losing specificity and vice-versa.

Myth: A reliable test is 100% accurate

Fact: Virtually every test has a degree of error. Therefore, it’s important to balance specificity and sensitivity

Is speed related to accuracy?

The sensitivity and specificity of a test do not necessarily depend on its speed of execution. Instead, these parameters depend on the technology used and many other variables. In principle, a faster test can be as accurate, or even more accurate, than a slower one. On the other hand, faster and cheaper tests often involve technologies that are newer than the golden standard and therefore need more work to assess and improve accuracy.

What is the acceptable accuracy for a test?

The accuracy of a new test is usually expected to be comparable to or better than the golden standard. However, there isn’t a fixed threshold to determine whether a test is accurate enough. Since sensitivity and specificity are often a trade-off, the choice may depend on the situation.
When screening travellers at an airport, for example, it may be more useful to identify as many carriers as possible with a first-line test (higher sensitivity and lower specificity) and eventually rule out the false positives with a second-line test which is less sensitive and more specific.
The sensitivity and specificity of a test must be properly measured and validated statistically. Validation is an essential part of making a new test and is critical to ensure reliability.
Different methods are useful for different testing strategies.
That is why the CORONADX project is developing and validating three rapid methods with different speeds, costs and accuracy that will hopefully fit various real-life situations.

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