Even good tests can give inaccurate results
Clinicians and researchers said that a number of factors could cause inaccurate results on COVID-19 tests, and many of them have nothing to do with the test’s design.
For starters, the timing of when a patient receives the test matters. “If you’re far out from the initial exposure, the more days you are after onset, viral load goes down,” explained Stanford’s Pinsky. Viral load refers to the amount of virus that is being emitted from an infected person’s cells, and if that drops too low, even a person who still has an active infection may test negative.
Another issue is where the virus is in a person’s body. As the disease progresses, scientists think the virus tends to move down into a patient’s lungs, so the window of time when a nose swab will return a positive result may be limited.
“One of the issues with this stupid virus is that, if it’s down in your lungs, and we’re putting a swab up your nose, that’s not the best way to measure what’s in your lungs,” said Alex Greninger, assistant director of the clinical virology lab at the University of Washington Medical Center.
While it is possible to stick a scope down a patient’s airway to collect a sample from the bottom of the lungs, this is a much more complex procedure that requires sedating the patient. Technicians can ask a patient to cough up phlegm, known as sputum, but doing so substantially raises the risk of infecting health care workers. Even with a nasopharyngeal sample collected with a nose swab, one needs to collect it properly, which involves sticking the swab quite far up a patient’s nose.
Daniel Brook, a freelance journalist and historian in New Orleans, says he thinks his test result may have been a false negative because he was incorrectly swabbed.
During Mardi Gras, he hung out with a friend who was visiting from Manhattan. A few days later, as he started to get night sweats and chills, Brook’s friend texted to say that he had tested positive for COVID-19. Brook has asthma, so when he started to have trouble breathing, he went to an urgent care center, which said it didn’t have enough tests to give him one.
Four days later, as Brook found himself even more winded going up stairs, he and his girlfriend, who also had symptoms, received a letter from an emergency room doctor that would get them a test at a drive-through center. They first were tested for the flu and then finally for COVID-19.
“This flu test was way the hell in there. It was almost like you ate too much hot pepper,” Brook said. “And then we had this COVID test, and it was barely in the nose at all, which may be one of the issues.” Nine days later, they received their results: Both were negative.
Brook was confused. He had been trying to tell all of the people he had been in contact with, like his barber, that they might have been exposed, and he shared the good news with many of them. But his doctor told him that clinically, he had all the symptoms of COVID-19, and that his diagnosis would not change based on his test result.
Even if the sample is taken correctly, mishandling of the swab can also invalidate the result. RNA is similar to DNA but due to chemical differences is a much more fragile material and degrades more readily. This coronavirus is an RNA virus, essentially a string of RNA encased in a membrane “envelope.”
Abbott, one of the test makers, said that it recommends that samples be kept for no more than 8 hours at about 60-85 degrees Fahrenheit, or refrigerated for 72 hours. “People should make sure it is tested in a timely fashion,” Abbott said in its statement to ProPublica.
None of this bodes well for the numerous labs that have reported backlogs of tens of thousands of samples that are waiting to be tested.
A technician at an academic laboratory, who asked for anonymity because he is not authorized to speak on behalf of his university, described seeing basic mishandling of samples that is probably ruining dozens of patients’ test results.
“I don’t know why, but with COVID, we’ve just been awash with problems,” he told ProPublica. “Even simple things like caps not screwed on tightly – we’ll get a bag of samples, and two or three of them will be leaking, so you have this media completely soaking the inside of the bag. If one of those leaking samples is positive, you’ll have droplets all over the bag.”
Those samples, the technician said, often can’t be processed at all. His experience isn’t unique: In Alabama in late March, hundreds of samples were ruined in transit to a lab in Montgomery.