False-negative SARS-CoV-2 RT-PCR tests occur less often than previously observed
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Although false-negative results from nasopharyngeal SARS-CoV-2 RT-PCR tests are possible, they occur less often than previously observed, according to study results published in Clinical Infectious Diseases.
“By characterizing the experience of two large U.S. health systems on the short-term occurrence of newly positive SARS-CoV-2 results among initially test-negative patients, we provide data on a topic of practical significance that should be used in combination with other reports to guide the use and interpretation of this common testing modality,” the researchers wrote.
Dustin R. Long, MD, of the department of anesthesiology and pain medicine at the University of Washington School of Medicine, and colleagues extracted SARS-CoV-2 RT-PCR test results, including 10,583 from the University of Washington and 12,543 from Stanford University, from electronic medical records. Tests were performed between March 2 and April 7, 2020 and test results through April 14, 2020, were obtained from the records to provide a 7-day observation period. Cycle threshold (Ct) data were taken from the appropriate lab information system.
The researchers found that 91% of all initial test results were negative. The majority of patients (including 95.9% of University of Washington patients and 97.4% of Stanford patients) did not receive repeat testing within 7 days and did not need later evaluation via outpatient or inpatient visits or the ED.
A total of 4.1% of University of Washington patients and 2.6% of Stanford patients had repeat testing despite an initial negative result. Most results, including 95.9% of University of Washington patients and 97.2% of Stanford patients, were again negative on repeat tests conducted within 7 days. A small percentage (3.5%) of all patients who were retested within 7 days were found to be positive, indicating that their initial result was a false negative.
“The cause of false-negative initial results also cannot be determined with confidence,” the authors wrote. “However, the range of Ct values observed in subsequent positive assays suggests that both sampling inefficiencies and low viral load (in cases of adequate sampling) may be contributing factors in this population. Additionally, it cannot be ruled out that some discordant test results in this cohort may be due to newly acquired infection.”
Perspective
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Amesh A. Adalja, MD
This study by Long and colleagues illustrates that, for most individuals, repeat COVID-19 testing is very low yield. The fact that only 3.5% of individuals retested within a 7-day period tested positive reassures us that a negative test can generally be relied upon. However, there are going to be clinical situations in which a repeat test may be indicated. Usually, there may be other ancillary information that suggests that COVID-19 is the true cause of symptoms. However, as we move through this pandemic, it will be important not to neglect other causes of infectious diseases that have symptoms similar to COVID-19 and not overly anchor ourselves to that diagnosis when testing is negative.
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