A negative test result isn’t proof you don’t have COVID-19
You can be certain you have an active COVID-19 infection if you test positive for it, but a negative COVID-19 test result doesn’t necessarily mean you don’t have the infectious disease.
The test being used by the Humboldt County Public Health Lab — the Centers for Disease Control and Prevention’s 2019-novel coronavirus real-time reverse transcription polymerase chain reaction diagnostic panel — is highly specific, meaning it will detect what it’s intended to detect and little else, and highly sensitive, meaning it will detect small quantities of that substance, which in this case happens to be the virus’s genetic code.
“It really is one of the gold standards,” Humboldt County Health Officer Dr. Teresa Frankovich said in a Wednesday video.
At the moment, polymerase chain reaction, or PCR, tests are the most widely used test to detect severe acute respiratory syndrome coronavirus 2, or SARSCov-2, the virus which causes COVID-19, because they’re highly accurate, said American Association for Clinical Chemistry President Dr. Carmen Wiley in a video.
“Even if there’s only a tiny amount of virus present in the patient sample, PCR … is able to take genetic material from the virus, target specific gene segments that only the virus has and make a very large number of copies of those segments so that they can be detected and measured,” Wiley said.
Because the CDC test specifically targets SARS-Cov-2, it is highly unlikely it will cross react with another genetically similar coronavirus and lead to a false positive result, when a person who doesn’t have the disease tests positive for it.
The same can’t be said of false negatives, when a person who
has the disease tests negative for it.
“Negative results do not preclude 2019-nCov infection and should not be used as the sole basis for treatment or other patient management decisions,” the CDC’s instructions for the test state.
False negative results could occur for a variety of reasons, ranging from the virus being below the limit of detection to when and where in the body the virus is being collected.
In a preliminary investigation of 1,070 samples collected from 205 patients with COVID-19 in China, lower respiratory specimens had the highest positive rate while nasal swabs had a 63% positive rate, though the sample size was only eight swabs, according to “Detection
of SARS-CoV-2 in Different Types of Clinical Specimens” published March 11 in the peer-reviewed Journal of the American Medical Association.
Bronchoalveolar lavage fluid, which is collected from the lungs, had a 93% chance of testing positive for the virus while coughed up phlegm, or sputum, had a 72% positivity rate, according to the survey. Other parts of the body had much lower positive rates, such as 1% for blood and zero for urine.
Currently the CDC recommends using nasopharyngeal swabs to test for COVID-19 because collecting samples from the lower respiratory tract requires “increased technical skill and equipment needs” and should only be done for people exhibiting severe symptoms, according to the agency’s instructions for collecting and handling specimen.
Samples collected from the upper respiratory tract
have different rates of testing positive depending on how far into an infection a person is, according to “Variations in False-Negative Rate of Reverse Transcriptase Polymerase Chain Reaction-Based SARSCov-2 Tests by Time Since Exposure” published May 13 in Annals of Internal Medicine.
The literature review found “the probability of a false-negative result in an infected person decreases from 100%” on Day 1 to 67% on Day 4. “On the day of symptom onset, the median false-negative rate was 38%,” decreasing to 20% on Day 8 and then beginning to rise again from 21% on Day 9 to 66% on Day 21, the review states.
“Remember that when you’re tested, it tells you your status for that day,” Frankovich said. “It really doesn’t tell us anything about your status for the next day or the following day.”
Health officials are looking at other factors besides positive test results to determine if a person has COVID-19, such as the kind of symptoms they have and whether they’re a known contact of a person who tested positive.
Frankovich said it wouldn’t be helpful to get tested more than once every two weeks if you don’t have symptoms.
“But if you develop mild symptoms, even if it’s been a short period of time, come on in and get tested,” Frankovich said. “If you develop more significant symptoms, contact your provider.”