Feature

Are we placing too much value on a negative test result?


Matt Woodley


23/07/2020 4:07:20 PM

Multiple state governments only require people with respiratory symptoms to continue self-isolation if they test positive for COVID-19, despite the risk of false negatives.

Person receiving nasal swab.
PCR testing sensitivity for coronavirus may be as low as 66% in clinical settings. (Image: AAP)

In laboratory conditions, polymerase chain reaction (PCR) testing for coronavirus achieves 100% sensitivity, meaning there is zero chance of returning a ‘false negative’.
 
However, multiple studies have subsequently shown that accuracy diminishes substantially in clinical settings, and as many as one in three coronavirus cases may be missed. This can be due to multiple reasons, from conducting the test incorrectly, to the time the swab was taken.
 
‘A negative test, whether or not a person has symptoms, doesn’t guarantee that they aren’t infected by the virus,’ Dr Lauren Kucirka, a resident physician at Johns Hopkins University and lead author of a recent study into PCR testing accuracy rates said.
 
‘How we respond to and interpret a negative test is very important because we place others at risk when we assume the test is perfect.’
 
As a result, health authorities should not rely on a single test to clear someone of coronavirus, especially if they are deemed at high risk of having the disease, according to Associate Professor Mark Morgan, Chair of the RACGP Expert Committee – Quality Care (REC–QC).
 
‘It is better for people who are symptomatic and might spread respiratory illnesses to self-isolate even if their COVID test is given the all clear,’ he told newsGP.
 
‘False negative tests are common if the test is done too early after a person has been exposed to infection. A negative test result could give a false sense of security and lead to someone inadvertently spreading COVID.’
 
Despite the threat of false negatives, only Western Australia, the ACT and Tasmania explicitly advise people to stay at home until respiratory symptoms subside, regardless of test results.
 
Conversely, Victoria, New South Wales, South Australia and Queensland health department information pages only advise self-isolation until receiving a negative result, while the Northern Territory offers no advice on what to do should this occur.
 
University of Sydney epidemiologist Associate Professor Ying Zhang told newsGP this situation represents is an issue, as there are patients who have recorded multiple negative results before a positive one.
 
‘It’s all about the likelihood of spreading the virus. In order to minimise the risks, it would be more cautious to suggest people stay at home until their symptoms have gone – and maybe longer,’ she said.
 
‘Ideally, if there’s a good reason for them to do the test it’s better stay at home for two weeks regardless of the result. But it could be too hard for many people.’
 
Warnings regarding false negatives were issued in Australia as early as April, while doctors elsewhere have also raised the alarm over the potential for false negatives. One medical group even accused the UK government of ‘evasion’ and a ‘wall of silence’ regarding how many swab tests result in false negative readings.

When approached by newsGP for clarification, a Victorian Department of Health and Human Services (DHHS) spokesperson said it is important ‘anyone who is unwell stays home’ until symptoms subside as it is ‘the best way to prevent spreading illness in the community’.
 
However, Victoria’s Chief Health Officer Professor Brett Sutton recently downplayed concerns surrounding false negatives, telling reporters that people who test negative are unlikely to spread the virus and people with ongoing symptoms should only ‘consider retesting’.
 
‘There’s no capturing people who have very low viral numbers and maybe they’re less infectious so it’s less of an issue,’ he said.
 
With Victoria in the midst of a major increase in cases – recording another 403 on Thursday 23 July – Premier Daniel Andrews has lamented the high rates of people not self-isolating in the time between developing symptoms and receiving test results. But his pleas to self-isolate have not extended to people who have tested negative but still have ongoing respiratory symptoms.
 
‘Unless we have people who get tested staying at home and isolating until they get their results, then we will not see these numbers come down,’ Premier Andrews said.
 
University of South Australia epidemiologist and biostatistician Professor Adrian Esterman told newsGP in areas with high community transmission any approach that does not require people with ongoing respiratory symptoms to stay home until they have passed is ‘not sensible’.
 
‘For all states and territories apart from New South Wales and Victoria, it doesn’t really matter,’ he said.
 
‘We’ve got little or no virus circulating, at least at the community level, therefore the rules they have about testing and symptoms won’t do much because we don’t have much in the way of virus.
 
‘But for Victoria and to a lesser extent New South Wales it is important. If you’re getting 20% of people with symptoms producing a false negative, then of course it’s an issue.
 
‘Victoria is currently plateauing at about 300–500 cases a day, and unless they take more action, it’s going stay there and it’s not sustainable.’

Brett-Sutton-article.jpgVictorian Chief Health Officer Professor Brett Sutton. (Image: AAP) 
 
According to Professor Esterman, if case numbers in Victoria do not go down soon, effective contact tracing will become impossible.
 
‘Somehow they’ve got to get those [daily] cases down to at least 250 or under, so they’ve got to take extra measures. One of them could be changing the rules about symptoms and testing,’ he said.
 
‘[They should say] that if you have respiratory symptoms, even if you test negative, please stay at home until the symptoms clear up. It makes sense because even if you’ve only got a cold – you don’t want to give it to others, anyway.’
 
Aside from stretching contract tracing capabilities, Associate Professor Mark Morgan said too many people not self-isolating could also place pressure on testing capacities.
 
‘People who catch coughs and colds have somehow been exposed to respiratory viruses indicating a failure of community social distancing and personal hygiene measures,’ he said.
 
‘It is not ideal to have people with infectious coughs and colds out and about in the community or in the workplace, because more and more people will need to go through testing.’
 
The continued increase in Victorian cases, and threats of a potential wave in NSW, mean these states should consider even more measures to limit the spread, Professor Esterman said.
 
‘Now’s probably the time they should change [self-isolation requirements] and some other things as well,’ he said.
 
‘For example, my understanding is that [in Victoria] you’re told to work from home unless you have to go to work. Now, to me, that’s way too loose. I would only allow people in absolutely essential occupations to go into work. Otherwise, you’re simply spreading it.
 
‘There are rules that they can change to try and tweak it and get the numbers down. They’re not huge changes, but they’re enough to start reducing it.’
 
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Dr Ian Mark Light   24/07/2020 9:01:04 AM

The viral spread is greatest indoors in crowded poorly ventilated spaces .
There is the hypothesis that social gatherings in the Melbourne winter is indoors - in the warmer states social gatherings occur far more outdoors .
To reduce spread the evidence is indicating physical distancing hand hygiene surface deep clean good masks and head visors and well ventilated spaces with clean fresh air .
Rapid turn around PCR and antibody tests are being developed and in urgent cases eg patients in hospital are being used now with very good sensitivity and specificity but there are not enough such tests for wider use at yet .


Dr Charles Raymond Ellis   24/07/2020 10:36:33 AM

Media regularly show swabs being done. I haven't seen one yet that wasn't a painful cribriform plate biopsy. No wonder so many false negative results. Please can the RACGP advocate for proper training of these testers so they can learn to do a (far less painful) correctly performed nasopharyngeal swab.
Also, rules in Victoria re self quarantine and management of COVID so lax it saddens me. Leadership there have (and will have) a lot of deaths on their hands of health and non-health persons . Elimination in Victoria and NSW needed now.


Dr Peter James Strickland   24/07/2020 11:18:13 AM

We now have a situation of "too many cooks inferring in the kitchen", and causing tunnel vision and panic among so-called experts. People with symptoms need to be tested for INFLUENZA as well ----and even though the present isolating, distancing and sanitising has caused a catastrophic fall in influenza cases to date. it is well known that OUTSIDE activities are far less likely to produce spread due to air dissipation and daylight UV (although UV-C is weak at ground level, but there is some positive eradication by UV-B). The problem now is the dictatorial level of governments and police with fines etc. --that causes resentment, anxiety, depression and increased suicides, and the total breakdown of human relations, businesses, jobs and hope. No positives are coming out here, and only panic every day, and it is time for pragmatic decisions, and to restore hope by giving fixed dates for return to normal travel and borders. Eradication is impossible, but control is here now.