Ashley Bloomfield webinar - response to follow up questions

28 September 2020

1. What is the sensitivity of oropharyngeal/ nasopharyngeal swabs?

Detailed information is available on the Ministry of Health website regarding the accuracy of COVID-19 tests, here. 

2. Please provide further research and clarity on:

a. Sensitivity of RT-PCR SARS-CoV-2 tests is estimated at around 70 - 80

  • Currently in New Zealand, the only publicly available diagnostic test is the polymerase chain reaction (PCR) test. This test is very reliable for diagnosing COVID-19 if it is taken at the right time (during the first week of the illness). 
  • According to recent studies, the analytical sensitivity to pick up the COVID-19 virus, if there is actually virus present in the specimen, is around 95%. This is based on comparing Real Time Polymerase Chain Reaction (RT-PCR) test results from swabs with more sensitive testing methodologies, such as whole genome sequencing.
  • A negative result can tell us that the person was unlikely to be infectious at the time of the test. When tests were done on samples without the virus, the tests correctly gave a negative result 96% of the time.
  • If you have symptoms of COVID-19, are tested less than 7 days since onset of these symptoms and test negative, then our pre-test probability of giving a false negative result is low enough to say with certainty that you don't have COVID-19.
  • If a person with a higher risk of having COVID-19, for example a known close contact of a confirmed case, returns a negative test, depending on when the test was taken and when the exposure occurred, they may still be asked to stay in isolation and have a repeat test at a later point to rule out infection.
  • With regards to the 30% false negative comment, this was a flawed study out of China which mentioned a 30% false negative rate of PCR. This appeared as a preprint (not peer reviewed) in MedRxiv on 17 February 2020 and has been widely quoted.

For more information consult

b. Sensitivity of oropharyngeal versus nasopharyngeal tests 

  • The sensitivity of oropharyngeal combined with sampling of deep nasal nares is similar to nasopharyngeal swab testing. Both Australia (refer link to PHLN guidance below) and New Zealand recommend either approach.
  • OPS are not as sensitive as NPS, or OPS plus deep nasal. Swabs that are not flocked will collect less cellular material. Together with just taking an OPS rather than OPS plus deep nasal or NPS, there are 2 variables that decrease sensitivity. How much this affects the diagnostic sensitivity depends on the time the specimen was taken in the illness and potentially whether the person is symptomatic or not.

For more information consult

3. At what age in a low risk child would you suggest swabbing orophryngeal/ nasopharyngeal?

  • There is no absolute age below which swabbing is not recommended. We have suggested that swabs are not routinely collected in MIFs in children under 6 months of age, however if there was a high exposure history e.g. close contact of a confirmed case, then it can be considered.

For more information consult: 

4. How is the prioritisation done at labtests in Auckland? It seemed like there was a lack of prioritisation in late August when we were asked to highlight the forms of certain specimens.

  • In the height of the recent outbreak samples from close contacts were the highest priority with other groups such as samples from HIS patients also prioritised highly.
  • The MoH worked with the Auckland labs to provide prioritisation of samples, however it relied on the samples being correctly indicated as a priority.

5. I heard you mention the complexities of the border workforce. Is there any differentiation between those higher risk (For example those at Jetpark) versus those in a MIF that has not seen any positive cases in terms of testing strategy?

Under the new COVID-19 Public Health (Required Testing) Amendment Order 2020, which came into effect at 11.59pm on 06 September 2020 routine mandatory testing has been further broken down for certain higher risk groups within the border workforce. Workers at Managed Quarantine facilities will be tested once every seven days, whilst workers at Managed Isolation Facilities will be tested once every 14 days.  In addition, workers transporting persons required to be in quarantine to and from the facility will be tested once every seven days, whilst workers who transport persons required to be in managed isolation to and from the facility will be tested once every 14 days. 

6. Rod Jackson mentioned a study showing 60% of 100 people (mean age 49yrs) with asymptomatic or mild COVID-19 illness developed myocarditis. Do you have any more info on this study?

The Ministry's Science and Technical team is currently engaged in research on the long-term complications of SARS-CoV-2 infection and what the implications may be. They will ensure to keep Dr Betty informed of this work and to communicate the information appropriately.