When is a Covid-19 test result a weak positive, why more of them are being found, and how the cases are dealt with

The Ministry of Health has set up an expert panel to work out the best way to deal with cases where people give what is being called a weak positive result when tested for Covid-19.

One such case was that of former Stuff reporter Tom Kitchin, now working for Radio New Zealand.

Kitchin first got sick with the virus back in March during the first outbreak. By May, he was cleared.

The former Christchurch-based reporter told Morning Report that recently he had a sore throat, felt unwell and tired – similar symptoms to what he experienced back in March – so he got tested for the virus and got a weak positive test result.

“The district health board rang me and I was in complete shock, I just could not believe it really,” he said on Radio NZ.

He has been in isolation, and his close contacts were asked to self-isolate and get a test. Those contacts have all returned negative tests, he told Morning Report.

Health officials believed he was showing remnants of the old virus he had back in March. The same day he returned a weak positive test, he was swabbed again and that test returned negative.

On Tuesday, Director-General of Health Dr Ashley Bloomfield listed four types of situations where testing was often resulting in such weak positives, which suggested an old infection.

In those cases the results had what Bloomfield described as a high cycle threshold or Ct value.

The Ct value is related to the polymerase chain reaction (PCR) process widely used to test for Covid-19.

According to the Centre for Evidence-Based Medicine at Oxford University, PCR works by amplifying genetic material exponentially, by doubling the number of virus molecules time and again.

The Ct value is the number of amplification cycles needed for the virus genetic material to become detectable.

Talking on Radio New Zealand on Wednesday, Bloomfield said that early in an infection the Ct value was lower, under about 25.

In other words, 25 or fewer amplification cycles would be needed for virus genetic material to be detected in those samples.

Around 30 “you start to wonder”, Bloomfield went on to say. “If it is over 35 then you are pretty confident this is an old infection.”

Bloomfield said earlier that international research had shown people with historical infections, weak positive test results and a high Ct value were not infectious.

Former Stuff reporter Tom Kitchin was diagnosed with Covid-19 back in March, cleared by May, then recently returned a weak positive.

On Radio NZ, he said that for now a precautionary approach was being taken in the case of weak positives.

“We treat them as if they are a positive infectious case initially, until we do the retesting.”

The main evidence that such weak positive cases were not infectious was a very good study done in South Korea where researchers followed up several hundred people who returned weak positive results, he said.

Contact tracing did not find anyone else infected by one of those cases.

“The scientific consensus is that this is viral fragments from the earlier infection that seem to be able to be detected particularly when someone gets another respiratory infection and perhaps because there is inflammation in the throat and the cells are being perhaps broken down a little,” Bloomfield said.

Director-General of Health Dr Ashley Bloomfield said more Covid-19 testing was being done “and it seems that we are seeing just a few more” false positives.

“That then releases these fragments of virus that are detected but it is also not uncommon to have a weak positive and then a negative test.”

Canterbury Health Laboratories clinical director of microbiology Dr Joshua Freeman said it was possible to detect genetic material a long time after infection for many respiratory viruses.

The question was whether the people involved were infectious. “That is something we are still learning about and we have to exercise some judgment around these questions,” Freeman said.

The Ct value was just one of many things taken into consideration when trying to work out if someone might be infectious. Some other factors included a person’s exposure history and whether they had antibodies for the virus.

The PCR process was incredibly sensitive when it came to detecting the virus, Freeman said.

With the weak positives it was thought people probably had bits of dead virus in membranes in the back of the throat that persisted for a long time after people stopped being infectious.

Some of those membranes could be picked up when a swab was taken from the back of someone’s nasal passage.

Researchers were still learning about how long people were infectious but pieces of the puzzle were slowly coming together, and more was known now than was known three months ago, Freeman said.

There could be a variety of reasons for having a high level of confidence someone who returned a weak positive was not infectious, although the reasons could vary depending on the case.

There was always a chance a weak positive could be infectious, which was why additional testing and investigations were carried out.

“It could be someone very early in the infection, they could be on the way up. Repeat testing should resolve that,” he said.

Whether someone who returned a weak positive should go into isolation was a judgment based on various factors, including the implications of getting it wrong.

“I don’t think there is a straightforward answer to that question. You need to always take a precautionary approach and that is what I think the ministry [of health] has been doing, appropriately,” Freeman said.

Weak positives were becoming an issue now because some people without symptoms, who may have had an infection earlier, were being tested.

“Sometimes if you keep testing people, sooner or later you might turn up a positive.

“It is a bit like dipping your net into a pond full of goldfish,” he said. “If you dip your net in enough eventually you are going to pull out a goldfish.”

In his briefing on Tuesday, Bloomfield said more testing was being done “and it seems that we are seeing just a few more” false positives.

“There is no doubt that during that period of March/April when we went into alert level 4 that there was other infection in the community that was not identified and reported then, but because we broke the chain of transmission it did not continue to spread.”