Inconsistent Covid-19 tests cause panic
She said the hospital then declined to perform her son’s operation due to her testing positive even though the boy was negative. Two days later, she received results of tests she did at a different lab. These tests came back negative.
Many Pretoria residents are worried about just how accurate Covid-19 test results might be after receiving apparently “inconsistent” test results.
Michelle de Jager, a mother from Zwavelpoort, recently shared her experiences in this regard with Rekord.
Her four-year-old son has immune defects, which meant he struggled to fight any bacteria or disease.
“If there is anything in the air, then he is the one to get it,” said De Jager.
“So for him Covid-19 would be extremely dangerous.”
The boy was scheduled for a bronchoscopy and drainage of fluid from one lung and, according to procedure, he and his mother were both tested for Covid-19.
Michelle said her son’s results came back negative and he was thus sent to the theatre.
“They, however, wouldn’t allow me to accompany him because my results weren’t back yet,” she said.
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“While I was waiting, I got an SMS stating that I was positive. I was shocked.”
She said the hospital then declined to perform her son’s operation due to her testing positive even though the boy was negative.
“It just didn’t make any sense. My son and I were sharing everything, we were sharing a bed at hospital.”
They were, however, sent back home and had to remain in isolation.
Two days later, she received results of tests she did at a different lab. These tests came back negative.
“My husband and I tested positive and later negative within a matter of only a few days,” local resident Irene le Grange also told Rekord.
Le Grange is the mother to a six-month-old baby, a home cardiac patient who was supposed to go for a check-up at a hospital in May.
As a precautionary measure, both Le Grange and her husband got tested for the coronavirus two days before their baby’s admission.
“The following day, my husband got an SMS from the lab saying he was negative,” she said.
A medical local expert believes there is an explanation.
READ MORE: Another Tshwane EMS station shut after Covid-19 case.
Clinical pharmacologist and medical practitioner at Intercare Woodhill, Dr Morné Strydom, said there was an explanation for this.
The most commonly used and gold standard test for Covid-19 diagnosis is the RT-PCR test.
The RT-PCR test is highly sensitive and specific towards genetic markers of the SARS-Cov-2 virus.
It is done by collecting nasopharyngeal or oropharyngeal swabs or from other respiratory tract secretion sources like sputum.
With the RT-PCR test, labs can only run the test of a swab sample/specimen that was submitted for testing.
The lab can only confirm whether the genetic material of the specific virus was detected or not detected from the swab specimen that was submitted for testing.
“Depending on the quality of the swab taken and other dynamic factors, it is possible to have the virus, but test negative on a swab,” he said.
“Your respiratory tract secretions, including the post-nasal drip secretions at the back of your throat is dynamic and will fluctuate during the day and night.
“The patient may be congested with blocked sinuses with no or very little drainage of upper airway secretions. At a different time of the day, there might be more free-flowing sinus drainage which may host more viral particles.
“The virus is mostly detected in airway secretions of the infected patient, who may be symptomatic or asymptomatic. The viral load may range from low to high concentrations, depending on the phase of infection and physiological factors as explained,” he said.
“We may take a swab at a time in the day when you do not shed a lot of virus in your secretions.
“So you could be swabbing at the back of the throat or nose, but not ‘catch’ any viral material on the swab for it was not present in detectable concentrations in those secretions at that point in time.”
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Another factor was swabs being taken from people shortly after having gargled, flushing their noses with nose sprays, drinking hot beverages, chewing bubblegum, sucking on throat lozenges or taking anything that could “clean or sterilise the throat” for that period around specimen collection – all minimising the chances of “catching” the virus (if onboard), despite of a good quality swab technique used by the healthcare worker.
“For example, you’ve had a nice warm cup of coffee and ‘burned’ your throat clean. Now lab A takes a swab in the back of the throat by collecting an oropharyngeal swab. Chances are good it will come back as ‘not detected’ or ‘negative’, resulting in a possible false negative,” he said.
“Now lab B may test through the nose by collecting a nasopharyngeal swab, thus, a different anatomical location and different technique.”
At the level of the nasopharynx, the chances are less for external agents to have a possible “cleaning effect”, like a warm cup of coffee would have on the throat, for example.
“Hence, the test may have a higher probability to detect the virus resulting in a positive test result.”
Strydom said the RT-PCR test is highly specific and sensitive for the SARS-Cov-2 virus, thus, the chance for “false positives” are very unlikely, unless the sample was contaminated.
“Sample contamination would be the exception rather than the rule. On the other hand, false negatives are very common, for reasons already explained.
“If I take a swab from a truly infected person and I was ‘lucky enough’ to get a good quality swab by collecting sufficient amounts of virus material in detectable levels at that point in time, the patient will most probably test positive,” he said.
“If you test negative with a follow-up test a day after or even on the same day, that does not invalidate the positive test.
“If you’ve tested positive, you are positive.”
He, therefore, advised anyone who has tested positive to self-isolate as per current protocol, and not waste money and resources on repeated “confirmatory tests”.
*This article has been amended
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