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1 in 5 COVID-19 Tests Give False-Negative Results, Studies Find

technician operating RT-PCR machine for COVID-19 tests

Pedro Vilela / Stringer / Getty Images

Key Takeaways

  • The main diagnostic tests that detect the SARS-CoV-2 virus can have extremely high false-negative rates.
  • Studies show that the best day to get tested is the eighth day of infection. 
  • Other diagnostic measures should be used in addition to RT-PCR tests to screen for COVID-19. 

A new report shows that one in five COVID-19 diagnostic tests produce a false-negative result, even when administered at the ideal time. As a result, researchers are cautioning against using reverse transcriptase-polymerase chain reaction (RT-PCR) tests as the sole source of diagnosis during the pandemic. RT-PCR tests are the most commonly-used diagnostic tests for the SARS-CoV-2 virus, which causes COVID-19.

According to the Johns Hopkins University report, the accuracy of the tests peaks at 80%. Accuracy rates appear to be contingent upon the stage of infection, with the highest percentage of false-negatives reported in the first four days of infection, when people are often asymptomatic. If testing occurs on the eighth day of infection—usually three days after symptom onset—results are more accurate.

“Most people don’t know when exactly they contracted the virus, so the fact that testing on a wrong day could produce a false-negative result is a considerable problem,” Ayah Rashwan, DMD, a New York-based pediatric dental resident who pivoted to treating COVID-19 patients in intensive care, tells Verywell. Rashwan was not involved with the Johns Hopkins Report. “This could lead to super spreaders who are rapidly spreading the virus and unknowingly getting their family members and friends sick.”

What Is a RT-PCR Test?

The RT-PCR test is a molecular-based test that detects traces of a pathogen’s genetic material—in this case, the genetic material of the SARS-CoV-2 virus. A false-negative result signifies a RT-PCR test’s inability to detect the virus in a person with an active infection.

Tests Detect Different Results on Different Days 

To gauge test accuracy, researchers at Johns Hopkins assessed data from seven prior studies, which examined 1,330 nasal swab samples taken from both hospitalized and non-hospitalized patients. The researchers then compared data from RT-PCR tests to patients' history of COVID-19 exposure and onset of symptoms.

From there, false-negative rates were evaluated on a day-by-day basis:

Day of Infection False-Negative Rate
Day 1 100%
Day 4 67%
Day 5 38%
Day 8 20%

After day 8, researchers say the likelihood of infection begins to increase again.

Other studies mirror this finding. One patient-led study found that people who got tested too late in their infection received negative results, despite exhibiting the same long-term symptoms as those who received positive results by testing earlier.

“This data implies that the timing of the testing really matters," Hannah Wei, a qualitative researcher who analyzed these patient survey responses, tells Verywell. "If you see that the symptoms are similar but the day of testing is off by even a week, it will lead to patients getting denied the treatment that they really need."

What This Means For You

If you think you've been exposed to COVID-19 or are experiencing symptoms, try to get tested around the eighth day of infection—or the third day of symptoms—if you are able to keep track. If you receive a negative result but are symptomatic, play it safe and isolate yourself until symptoms resolve.

Why RT-PCR Tests Are Not Always Accurate 

The researchers at Johns Hopkins say one reason for a high number of false-negatives results could be the variability in viral shedding, which is the rate the virus leaves your body after when it can no longer reproduce.

Faulty sample collection techniques may also be to blame, which can happen if a clinician doesn't swab deep enough to collect the necessary samples. 

Some test kits possess faulty reagents—the chemicals used in tests to help detect SARS-CoV-2. In a press conference, Nancy Messonnier, MD, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention (CDC), explained that tests may not always work as well in other labs as they do in CDC labs, leading to inaccurate or inconclusive results.

To circumvent this issue, the CDC is taking extra measures to ensure high-quality reagent manufacturing. 

How Hospitals Are Working to Prevent False-Negatives

These false-negative test results occur both in lab and hospital settings. 

Amira El Kholy, APRN, an ICU nurse practitioner who works at multiple hospitals throughout Miami, Florida, says her hospitals aren't just relying on one test result; they're administering three different tests on patients experiencing COVID-19 symptoms:

  • Abbott rapid test: Displays results in a few hours
  • RT-PCR test: Takes about three days to process
  • Antibody blood test: Helps determine whether a patient has an active infection or previously had an active infection 

According to El Kholy, the Abbott rapid tests are extremely faulty, so if a patient is clearly exhibiting COVID-19 symptoms, they will be kept in isolation until RT-PCR results come back. If those results also come back negative but clinicians believe a patient has contracted the SARS-CoV-2 virus, they will perform the RT-PCR tests an average of three more times in order to obtain a positive result. 

“If a patient is positive, we know for sure to put them in isolation. If they’re negative, we still isolate them and wait to further confirm their results," El Kholy tells Verywell. "It’s important to isolate a patient if they have symptoms that are consistent with COVID-19, despite a negative test result."

Other Diagnostic Tools

The authors of the Johns Hopkins report aren't the only scientists emphasizing that different diagnostic measures should be used to supplement RT‐PCR tests. Researchers from Wuhan, China, for example, suggest the use of CT scans to both help diagnose COVID-19 and monitor a patient's progress.

According to El Kholy, blood tests that check for indications of inflammation—including C-reactive protein, creatine phosphokinase, D-Dimer, and lactate dehydrogenase—would be useful and simple additions to the diagnostic process.

Other experts recommend taking samples from a patient’s lower respiratory tract, rather than relying solely on the upper respiratory tract samples typically used in RT-PCR tests.

“Despite these findings, we still encourage the use of RT‐PCR tests because they’re important for testing, tracing, and curbing the rates of COVID-19,” Rashwan says. “It’s just important to be cognizant of the limitations of them and employ other diagnostic measures.” 

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