Being swabbed for the coronavirus can cause severe nosebleeds, a report has flagged.
Anyone who develops the infection's tell-tale fever, new and continuous cough, or a loss of taste or smell should be swabbed via the "gold standard" polymerase chain reaction (PCR) test.
In a bid to control asymptomatic transmission, people in England are also being urged to undergo a rapid – but less accurate – at-home lateral flow test twice a week.
While both tests are safe in the vast majority of cases, medics from Helsinki University have reported eight patients were admitted to their hospital's ear, nose and throat (ENT) unit with "complications" following a coronavirus swab.
Of these, four endured "nasal bleeds", two of which were "potentially life threatening".
One of the patients even developed sepsis, while three of the four required blood transfusions.
The remaining four patients had swabs broken off in their nose, but "fared well" overall.
Swabbing the nose and throat for coronavirus particles "is considered safe, despite adjacent vital structures", like the base of the skull, the Helsinki medics wrote in the journal JAMA Otolaryngology – Head & Neck Surgery.
Nevertheless, "single case reports and clinical observations indicate the possibility of severe complications".
The medics have now reported eight patients – aged 14 to 78 – were admitted to their ENT unit after being swabbed for the coronavirus between 1 March and 30 September 2020.
Over these seven months, more than 643,000 PCR coronavirus swabs were performed among Helsinki University's "catchment population" of 1.6 million people.
The "frequency of complications requiring treatment" in A&E was therefore calculated at 1.24 per 100,000 coronavirus tests.
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The eight patients, all of whom tested negative for the coronavirus, endured complications "immediately after" being swabbed.
One who developed a nasal bleed was pregnant, with the ordeal leading to "foetal risk", according to the medics. It is unclear if the bleed affected her pregnancy.
Another developed a Staphylococcus aureus infection, which led to sepsis. The patient is thought to have caught the bacteria from the "repetitive nasal packings" they were treated with.
Two of the four patients also developed dangerously low haemoglobin counts; the cells that transport oxygen around the body.
In men, a low count is generally considered to be under 13.5 grams of haemoglobin per deciliter (g/dL), while women should have more than 12 g/dL.
It is unclear what sex the four patients were, however, two of their counts were recorded at 6.4 g/dL.
As well as the nasal packings, the bleeds were also treated with medication and surgery.
All the patients are thought to have recovered.
The medics have stressed testing is "important" amid the pandemic, with the risk of a swab-related complication being "extremely low".
All eight patients are said to have endured complications due to an "incorrect sampling technique", like "excess use of force".
"Sampling should always be performed bearing in mind the anatomical structures of the nasal cavity and its surroundings to ensure safe sampling and correct results," wrote the medics.
"Force should never be used, especially in patients with known prior operations of the nose or skull base.
"The sampling swab should be directed along the nasal floor, not too laterally nor too cranially, until resistance is encountered."
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