What are ‘probable’ cases and why are they rising?

On Sunday there were nearly as many probable cases of Covid-19 as confirmed cases. But what is a probable case, and why are the numbers of probable cases climbing?

When probable Covid-19 case numbers were first shared on March 24, they represented 7 percent of the new cases reported to the media. On Sunday they accounted for 46 percent of the day’s new cases. 

Cases described as ‘confirmed’ are those a laboratory test has come back with a positive result. 

Director-General of Health Dr Ashley Bloomfield explained the definition of a probable case when he first introduced the term to the daily media briefings:

“This is a person who has returned a negative laboratory test result but the clinician looking after the person has diagnosed them as a probable case due to their exposure history and their clinical symptoms. These cases are actually treated as if they were a positive laboratory confirmed case and the actions taken are the same as for a confirmed case. That is self-isolation and active contact tracing.”

Data source: MoH daily media briefings

The recent spike in probable cases may not have anything to do with laboratory tests giving negative results for more people who are displaying symptoms. 

The case definition was recently changed.

If you live with a confirmed case and develop symptoms, you aren’t tested and are automatically considered to be a probable case.

The Ministry of Health’s case definition from April 3 reads:

“Household and other close contacts of those who have tested positive, and who go on to develop symptoms should not be tested unless they meet one of the red flags criteria below or are a healthcare worker. Otherwise, as above, if well enough they should be considered a probable case and should isolate and be managed at home with monitoring.”

How tests can return a negative result

There still may be false negatives in the mix. Newsroom has asked the institute of Environmental Science and Research for a guide on roughly how many are occurring.

University of Otago Associate Professor James Ussher explained how false negative results - where a person who has the virus tests negative - can occur.

“Viral shedding peaks around the time symptoms start and then declines. The amount of virus, especially in those with pneumonia, may be higher in the lungs than in the upper respiratory tract. Therefore, if the patient is tested too late in the course of disease the PCR may be falsely negative (ie the patient is infected but no viral RNA can be detected in the samples). Also, sometimes, the upper respiratory tract samples may be negative when the sputum is positive.”

Serological testing is not available in New Zealand yet. This is a test for antibodies - the immune system’s response - to the virus. It’s more use in finding out whether somebody had the virus than establishing if they’re in the early stages. 

“The immune response usually becomes detectable during the second week of illness. It is not useful for diagnosing infection during the first week as the immune response is not yet detectable.”

Apples with apples

When comparing the case numbers in other countries to New Zealand, it’s important to note most of the quoted numbers are of confirmed cases. While media in New Zealand are using the combined total of 1106, the total of confirmed cases from yesterday stands at 911.

The World Health Organisation, which collates a daily situation report, only reports on confirmed cases where there has been a positive laboratory result.

Depending on each country's ability to test, and policy on testing the total cases could be different to the confirmed cases. 

Disparities between confirmed and probable numbers can be seen clearly in the deaths attributed to swine flu. Officially around 18,000 deaths were attributed to the pandemic as only fatalities confirmed with a laboratory test were included. In countries where there was no access to health facilities to test, deaths weren't recorded. The estimated number of deaths caused by swine flu is 284,500, with some estimates of over 500,000. 

Can you help our journalists uncover the facts?

Newsroom is committed to giving our journalists the time they need to uncover, investigate, and fact-check tough stories. Reader donations are critical to buying our team the time they need to produce high-quality independent journalism.

If you can help us, please donate today.


Newsroom does not allow comments directly on this website. We invite all readers who wish to discuss a story or leave a comment to visit us on Twitter or Facebook. We also welcome your news tips and feedback via email: Thank you.

With thanks to our partners