COVID-19 test: at-home rapid tests inaccurate

Viral load differences can lead to false negatives with nasal antigen tests.


The emergence of rapid at-home COVID-19 tests has offered individuals a convenient and accessible means of detecting the virus in the comfort of their own homes. However, concerns have been raised regarding the accuracy and reliability of these tests in accurately identifying infections.

This study aims to investigate the potential limitations of at-home rapid COVID tests and determine how much they may miss positive cases. By evaluating the performance of these tests, we can gain valuable insights into their effectiveness and inform public health strategies for efficient and accurate virus detection.

New research from Caltech reveals that rapid COVID-19 tests using nasal swabs can frequently yield false negatives, incorrectly indicating that an individual is free from infection despite the presence of the virus in other parts of the respiratory tract.

The study, led by Rustem Ismagilov, Ethel Wilson Bowles, and Robert Bowles, Professor of Chemistry and Chemical Engineering, focused on tracking viral loads in the nose, throat, and mouth during a COVID-19 infection. Surprisingly, the findings demonstrate significant variations in virus levels across these regions, challenging the assumption of uniform viral distribution. The research outcomes were published in the journal Microbiology Spectrum on June 15.

Alexander Viloria Winnett, biology graduate student, and study co-author, said. “Generally, we saw that most people have virus first appear in their throat and saliva, and then, sometimes days later, in their nose. Each sample type from a single person follows its distinct rise and fall of viral load, making a big difference in which sample type is used for testing.”

During the early stages of the pandemic, deep nasal swabs performed by medical professionals were considered the gold standard for COVID-19 testing, although they were uncomfortable and time-consuming. At-home nasal rapid antigen tests gained popularity as the pandemic progressed due to their convenience and quick results.

However, a recent Caltech study discovered a notable delay in detecting the virus in the nose compared to the throat or saliva. Among the 17 study participants, 15 had high and presumably infectious virus levels in their throat or saliva before obtaining a positive antigen test result.

Surprisingly, one participant had extremely high virus levels in throat swabs for almost two weeks, while nasal swabs remained undetectable or required a susceptible PCR test for detection. Natasha Shelby, the study’s administrator, explained that rapid antigen tests have limitations in testing only the nose and having low sensitivity, challenging the assumption that they consistently detect infectious individuals.

The recent Caltech COVID-19 study has revealed that at-home rapid tests can provide false negatives, indicating a person is virus-free when infected and potentially still contagious. The low sensitivity and limitation of nasal swabs contribute to delayed detection of infections. While rapid tests have advantages such as quick results and affordability, the findings emphasize the need for testing kits that sample both the nose and throat. Such tests, commonly used in other countries, could improve the effectiveness of detection strategies.

The study’s lead researcher, Viloria Winnett, hopes the findings will inform responses to COVID-19 and future respiratory viruses. The reasons behind the differing viral trajectories in different parts of the body remain an open question for further research. The study received primary funding from Caltech’s Ronald and Maxine Linde Center for New Initiatives and Caltech’s Jacobs Institute for Molecular Engineering for Medicine.

In conclusion, the study demonstrates that daily nasal antigen tests can miss infections and fail to detect individuals who are presumably infectious. The variations in viral load among different specimen types, particularly the delay in detecting the virus in the nose compared to the throat or saliva, contribute to the limitations of rapid antigen tests. The findings emphasize the need for improved testing strategies and the development of testing kits that can sample multiple sites to enhance the accuracy of COVID-19 detection.

Journal Reference:

  1. Alexander Viloria Winnett, Reid Akana et al. Daily SARS-CoV-2 Nasal Antigen Tests Miss Infected and Presumably Infectious People Due to Viral Load Differences among Specimen Types. Microbiology Spectrum. DOI: 10.1128/spectrum.01295-23