COVID-19 testing: What you need to know about test accuracy

April 20, 2020

The global diagnostics community has raced to meet the unprecedented need for tens of millions of tests for SARS-CoV-2, the novel coronavirus that causes COVID-19. In addition to our cobas® SARs CoV-2 test, many tests from other diagnostic companies have received Emergency Use Authorization (EUA) by the U.S. Food and Drug Administration (FDA), greatly improving our national testing capacity. No one company could come close to fulfilling the ever-growing global need for tests.

Given these tests were developed quickly and under emergency circumstances, it’s no surprise that people have questions and concerns about their accuracy. 

You can think of test accuracy in two ways:

  1. If I have the disease, will the test correctly identify it? This is known as test sensitivity, or a true positive rate.
  2. If I don’t have the disease, will the test correctly show that? This is known as specificity, or a true negative rate.


One thing to keep in mind is that no diagnostic test offers perfect performance. With any test, there will always be some false positives (the test says you have the disease when you actually don’t) and false negatives (the test says you don’t have the disease when you actually do).

To develop our test, we leveraged years of experience in creating high quality molecular tests. PCR (polymerase chain reaction) technology, which we use in our cobas SARS-CoV-2 test, is considered the gold standard for detecting the presence of the virus. We’ve striven to achieve high specificity and high sensitivity in our test, which is critical for the accurate diagnosis of COVID-19.

The limit of detection (LoD) is an important performance characteristic of clinical laboratory diagnostic, therapy monitoring and screening tests. LoD is the lowest quantity of a substance that can be distinguished from the absence of that substance with a stated confidence level. Studies have demonstrated that our test is able to detect very low levels of the SARS-CoV-2 virus with 95 percent certainty, and it does not detect other respiratory pathogens.

We expect, based on previous experience with similar PCR technologies to detect viruses, that the frequency of false positives and false negatives will be low.

There are several factors independent of accuracy that can lead to false positives or false negatives with any test, including our PCR-based test. Some of these include:

  • The sample is collected too early. Newly infected individuals may have low concentrations of the virus present at the time of sampling. To detect the presence of SARS-CoV-2 using a PCR-based test, which amplifies and analyzes the genetic code of viruses, the virus must be present in high enough concentrations. If a test is performed too early in the infection period (typically, day 1-4), it may come back negative even though the patient does have the virus.
  • The sample is collected too late. If a sample is taken after the disease has progressed too far, the amount of virus that remains in the body might not be enough to be detected by the test, leading to a false negative result. (There are other types of tests, such as antibody tests, that can detect the body’s immune system response to the virus later, but they are not as effective at detecting the virus early in the disease progression.)
  • The sample is contaminated. Handling samples taken from patients who have high concentrations of virus brings a greater risk of contaminating other surfaces in the healthcare environment (such as gloves, sample collection devices, etc.). This could lead to contamination of other patient samples so they show up as false positives.
  • The sample is stored too long. Swab samples taken from patients have limited stability over time. If extended storage, transport, or handling causes a long delay before the sample is tested, it can be harder to detect the virus, leading to a false negative result.

Accurate testing plays a critical role in slowing the spread of COVID-19. We will continue to work with laboratories around the world during the pandemic to collect data on the use of our test to ensure that its performance is consistent with our other PCR-based tests and to give healthcare providers and their patients confidence in their test results.