From 2020 / 2021:
A PCR test is amplifying samples through repetitive cycles. The lower the virus concentration in the sample, the more cycles are needed to achieve a positive result. Many US labs work with 35 to 45 cycles, while many European labs work with 30 to 40 cycles.
The research group of French professor Didier Raoult has recently shown that at a cycle threshold (ct) of 25, about 70% of samples remained positive in cell culture (i.e. were infectious); at a ct of 30, 20% of samples remained positive; at a ct of 35, 3% of samples remained positive; and at a ct above 35, no sample remained positive (infectious) in cell culture (see diagram).
This means that if a person gets a “positive” PCR test result at a cycle threshold of 35 or higher (as applied in most US labs and many European labs), the chance that the person is infectious is less than 3%. The chance that the person received a “false positive” result is 97% or higher.
As a companion to today's academic paper and Substack article regarding PCR and scientific due diligence.
In New Zealand, this OIA request shows 40 cycles was used.