By Charles Coles: It’s a widely-held view among those who understand these things that when it comes to testing a covid swab using the ‘gold standard’ PCR test used by our government, obtaining a false positive (wrong) result is too easy.
It’s easy to obtain a result that’s inaccurate because the PCR test uses an amplification method that magnifies the swab sample as big as you want – just keep turning up the dial. It means that if a sample is magnified too much you can find anything you want to.
Because of this, the developer of the PCR test – Kari Mullis – advised that the amplification had to be kept to a low threshold – they call it ‘cycles’ – for a meaningful positive result.
In short, anything above 30 cycles will tend to deliver a false positive, an inaccurate result that can’t be relied upon to reflect a case of concern. Lower than 30 and you will get closer to an accurate result.
In New Zealand, the people working in the laboratories testing the covid swabs are using 40 cycles of magnification.
It means that the teeniest weeniest microscopic imperceptible fraction of anything can be detected. And surprise surprise, testing facilities here can find covid. Even if the person the sample was taken from feels perfectly well and are more than likely not pre-symptomatic or even assymptomatic. It’s sometimes called a weak positive.
The Buzz asked New Zealand’s Ministry of Health a few questions about the PCR tests it’s carrying out…
Q: The Corman-Droston report on PCR testing states that when it comes to covid-19 and the PCR test, anything above a testing regime of 30 cycles will produce a questionable result, and that anything above 35 cycles will produce 97% false positives.
Give the MoH is testing at 40 cycles, does the MoH have any plans to review its testing procedure in light of this report or to test at the lower cycle threshold to obtain a higher rate of accuracy?
A: No, all diagnostic laboratories are aware of the possibility that non-specific off-target amplification can occur beyond 37 cycles which then generate non-specific amplification curves. Therefore, all weak positive signals are viewed with caution and the test repeated from sample using another RT-PCR to exclude any contamination or off-target binding issues.
Q: Who decided that 40 cycles was the appropriate test measure?
A: PCR assays for all Covid-19 testing typically runs for 40 cycles in our accredited New Zealand Laboratories. The number of cycles is stipulated by the manufacturers who have performed extensive validation and verification of their assay.
Q: Could the MoH review all its PCR test results and release a more accurate picture of positive covid cases in New Zealand?
A: No, the Ministry of Health will not be reviewing all the PCR test results.