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Diagnostic Performance of an Antigen Test with RT-PCR for the Detection of SARS-CoV-2 in a Hospital Setting — Los Angeles County, California, June–August 2020

Our take —

The Quidel SARS-CoV-2 rapid antigen test has high specificity but lower sensitivity in both patients admitted to the hospital through the emergency department with COVID-19-compatible symptoms as the chief complaint (72%) and without (61%), enabling a quick way to detect most infected patients. However, negative antigen test results, especially in those with confirmed COVID-19 symptoms or other high prevalence conditions, should be followed by confirmatory RT-PCR testing to prevent false-negatives.

Study design

Cross-Sectional

Study population and setting

Between June 30 and August 31, 2020, matched samples (nasal, nasopharyngeal, and saliva) were collected from 307 patients with COVID-19-compatible symptoms listed as the chief complaint and 1,732 patients without (median age of total population: 56 years, 55% female) who were admitted to the hospital through the emergency department from a tertiary medical center in Los Angeles. The performance of the Quidel Sofia 2 SARS antigen fluorescent immunoassay was compared to a gold standard Fulgent COVID-19 RT-PCR test.

Summary of Main Findings

In those admitted to the hospital from the emergency department in a period of high COVID-19 community prevalence, 8% of patients without COVID-19-compatible symptoms as chief complaint and 39% of patients with COVID-19-compatible symptoms as chief complaint had a positive result from either test. Relative to the RT-PCR test, the Quidel rapid antigen test had a sensitivity of 61% in those without COVID-19-compatible symptoms as chief complaint and 72% in those with COVID-19-compatible symptoms as chief complaint. The specificity of the test was high (99.5% in patients without and 98.7% in patients with COVID-19-compatible symptoms as chief complaint). The results further confirm that rapid antigen tests produce results more promptly, are cost-effective, and highly specific, although they are less sensitive than RT-PCR tests.

Study Strengths

This study is one of the first and largest to report on the performance of the Quidel rapid antigen test in the clinical setting.

Limitations

In the study, patients were termed “symptomatic” or “asymptomatic” based on whether they had COVID-19-compatible symptoms listed as the chief complaint on admission to the emergency department which likely resulted in misclassification, as those with the chief complaint that was not suggestive of COVID-19 could still have had milder symptoms associated with the infection that were not recorded. Additionally, the study did not evaluate potential differences in rapid antigen performance by subgroups, such as by age (which ranged from 16-107 years). The study population was drawn from a single hospital in Los Angeles and only included those hospitalized from the emergency department, which may limit the generalizability of these findings to other clinical settings or the general population.

Value added

This study adds to the body of literature that shows rapid antigen detection tests have strong specificity yet modest sensitivity in COVID-19 patients, showing that positive results are generally correct while negative results may require further confirmatory RT-PCR testing, especially in high-COVID-19 prevalence conditions (such as patients entering the emergency department with COVID-19-compatible symptoms as their chief complaint).

This review was posted on: 4 September 2021