What your doctor is reading on Medscape.com:
MAY 16, 2020 — Recently, a patient arrived at the UC San Diego Health medical center with what are now classic symptoms of COVID-19: a history of coughs, pneumonia, and severe respiratory distress that required immediate intubation.
What he didn’t have was a positive SARS-CoV-2 test — neither the first nor the second time clinicians swabbed the back of his throat. SARS-CoV-2 is the virus that causes COVID-19.
“The two negative tests didn’t convince anybody,” said Davey Smith, MD, a virologist and chief of the division of infectious diseases and global public health at UC San Diego School of Medicine. It was only on the third test, when they sampled fluid from a bronchial wash, that they were able to find the virus.
Smith’s patient is not alone. Though almost all experts agree that broad testing for SARS-CoV-2 will be critical to understanding, containing, and eventually treating COVID-19, the effort is hampered by limitations of current tests.
The tests today, experts say, are so new that it’s unclear how reliable they are. Anecdotally, clinicians report false negative rates of anywhere from 2% to 30%, depending on what part of the body is being tested and what means they are using to get a sample, as well as epidemiological and clinical factors.
And the US Food and Drug Administration issued an alert earlier this week warning of false negatives with one of the most commonly used tests, Abbott Labs’ ID NOW rapid test for COVID-19.
Data published earlier this week in the Annals of Internal Medicine show that test accuracy varies widely over the course of the disease in a mixed population of inpatients and outpatients. On the day symptoms appear, the median false negative rate was 38%. That figure dropped to 20% on the third day after symptom onset, but climbed again to 66% about 2 weeks later.
But test results should only be part of the picture. The key is clinical suspicion informed by all the above factors, said Joshua Metlay, MD, PhD, chief of the division of internal medicine at Massachusetts General Hospital, and coauthor of a series of articles on clinical decision-making in the Annals of Internal Medicine.
“How we treat patients is going to depend on understanding this concept,” Metlay told Medscape Medical News. “It isn’t one number. It’s actually much more complicated and very nuanced.” If clinicians don’t understand that, he added, “We’re really going to make mistakes about how to use all these negative tests.”
When Hope Outstrips Reason
A positive SARS-CoV-2 test sets off a cascade of actions, in and out of clinical settings: In patients with symptoms, it triggers a set of protocols, as recommended by the National Institutes of Health and individual hospitals, around use of personal protective equipment (PPE) for staff, whether patients are placed in rooms with others or singly, and specific treatment choices, such as which ventilator protocol to use. By contrast, a negative test, in an ideal situation, should lead a clinician to keep looking for a causative agent or underlying problem. Quality care, in other words, relies on accurate diagnosis.
In patients without symptoms, a positive test means suggesting quarantine and isolation for two weeks, said Colin West, MD, PhD, professor of medicine and biostatistics at the Mayo Clinic in Rochester, Minnesota.
But because of the relatively high rate of false negatives, a negative test in an asymptomatic person can’t confer the kind of relief patients, the public, or policymakers would like it to, West said.
“People can’t relax their physical distancing, their handwashing, their surface hygiene, their mask-wearing” even with a negative test, he said, because they still could be carrying the virus.