COVID-19 PCR Testing and Cycle Thresholds

There is a rumour going around Instagram and other social media sites that The World Health Organisation has confirmed that the PCR test is flawed and all tests with a high cycle threshold (Ct) are invalid. This is untrue and Pearl Chemist Group is here to clear up the confusion once and for all, helping with understanding some of these COVID-19 terms.

What are cycle thresholds?
Here is how a PCR test is processed;  the viral genetic material on the swab is purified, stabilised and concentrated.  If there IS indeed material of interest in the sample, it is copied over and over by heating and cooling the material in the presence of various substances.  This process, each time it is performed, is known as a thermal cycle.

The genetic material is then amplified, which is called fluorescence. This process varies by lab, but essentially, a substance is added to each thermal cycle to make the genetic material release fluorescent particles. The point at which they become fluorescent (the number of thermal cycles it takes to become fluorescent) is known as the cycle threshold.

The fewer cycles it takes to become fluorescent, the higher the concentration of the viral genetic material is present. In other words, the fewer the cycles, the more virus there is.

What is the controversy?
There has been some backlash in that some people point to a higher cycle threshold as a flaw in PCR testing, making it invalid.

Public Health England says a positive test at higher cycle thresholds does normally represent someone with a lower concentration of virus when the test was done.

The WHO has reiterated previous guidance promoting PCR testing and states:

“...careful interpretation of weak positive results is needed (1). The cycle threshold (Ct) needed to detect virus is inversely proportional to the patient’s viral load. Where test results do not correspond with the clinical presentation, a new specimen should be taken and retested using the same or different [nucleic acid testing] technology.”

What is the takeaway?

  • The idea that positive tests with high Ct values are invalid should be dismissed as overly simplistic.
  • PCR Tests remain the gold standard for diagnosing an active COVID-19 SARS infection
  • Typical cycle thresholds are up to 40, many positive samples will be detectable at lower levels - they vary
  • Cycle thresholds may be indicative of stages of infection or infectiousness; further study is needed.

Pearl Chemist Group has PCR swab tests and fit to fly certificates for travel available at our vaccination clinics in the UK.  Visit us online to book in today.  www.pearlchemistgroup.co.uk