PCR cycle threshold (11-37) and positive cell culture (black line, 100% to 0%) (Jafaar/Raoult)
Already in mid-March, SPR explained that the highly sensitive PCR tests are prone to producing false-positive results and their individual predictive value may easily drop below 50%.
The issues with PCR tests are numerous:
- There can be large-scale test kit contamination, as both the US and the UK (and several African countries) discovered during the early phase of the pandemic.
- There can be testing site or lab contamination, which has led to countless false positive results, school closures, nursing home quarantines, canceled sports events, and more.
- The PCR test can react to other coronaviruses. According to lab examinations, this happens in about 1 to 3% of cases if only one target gene is tested, as is the case in many (but not all) labs and as the WHO itself has recommended to avoid ambiguous positive/negative test results.
- The PCR test can detect non-infectious virus fragments weeks after an active infection, or from an infection of a contact person, as the US CDC confirmed.
- The PCR test can detect viable virus in quantities too small to be infectious (see below).
If the virus is not widespread in a population, and there is no test kit or lab contamination, and labs test for at least two target genes, the risk of a false-positive result is low. This is why, for instance, New Zealand at times had no positive PCR tests for weeks despite frequent testing.
But if there is an ongoing infection wave, or if there has been a recent infection wave, or if labs test only for one gene sequence or struggle with contamination, things get more complicated.
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 37 to 40 cycles,