Recently, some websites and Internet users have reported that the World Health Organization (WHO) “admitted that the number of positive cases for COVID-19 is overestimated” and that the “new” CRP test is invalid. The reports appear to result from an announcement made by the WHO, advising laboratory technicians and test scientists to carefully validate the results of the PCR and to ensure that each test is carefully considered. Written in very technical language – aimed at PCR scientists and experts in the field – it is understandable how many misinterpreted the writing, but what is said in the ad is not necessarily what reached the public.
PCR test
First, let’s explain what a PCR test is. The Polymerase Chain Reaction, or PCR, is simply a method to duplicate the genetic material in a sample to make more of it, so that more tests can be performed. The test goes through different stages that are repeated up to 50 times, each doubling the DNA / RNA exponentially – the less genetic material in the sample, the more cycles are needed to reach a high enough concentration.
Scientists usually perform PCR when they have a sample of DNA or RNA and look for specific genes in that sample, but the amount of genetic material is very small. Searching a small sample of DNA for a specific sequence without PCR would be like trying to play Where is Waldo with a really blurry image – you need to make it clearer to find what you’re looking for. PCR is our most powerful method of doing this.
But how does this relate to COVID-19? To detect the presence of SARS-CoV-2 in a patient, the test removes a swab from the throat, which will contain virus particles if the person is infected. A special form of PCR, called reverse transcriptase PCR (rt-PCR), is then used to amplify the amount of genetic material in the sample before being tested for the presence of virus-specific genes.
WHO Confusion
PCR tests have been in place to check for COVID-19 infection for almost a year and have been our best method for understanding the prevalence of the virus. However, the WHO recently released a report on how the test results should be carefully examined to ensure its accuracy, and some felt that the test is inaccurate.
This is the first of two passages written by WHO:
“WHO guidance The diagnostic tests for SARS-CoV-2 state that a careful interpretation of weak positive results is necessary (1). The cycle limit (Ct) required to detect the virus is inversely proportional to the patient’s viral load. When the test results do not match the clinical presentation, a new sample must be collected and retested using the same or different NAT technology. ”
This passage simply requires that any positive test result that gives a weak signal be carefully examined to ensure that the result is a true positive and not a false positive, and if the patient has symptoms of COVID-19, repeat the test to be sure absolute result. Probably written as a reminder or an update to its official protocol, this procedure is already done in many COVID-19 test sites and in no way indicates that the PCR test is inaccurate.
It also talks about the cycle limit (Ct), which is the number of PCR cycles required to make the signal strong enough to be recorded. Some people claim that a large number of cycles will give a positive result for anything – this is not true, more cycles are needed when the concentration of genetic material is very low, as in the COVID-19 test. More cycles cannot highlight the presence of something that is not there.
The second passage states the following:
“WHO reminds IVD users that the prevalence of the disease alters the predictive value of test results; as the prevalence of the disease decreases, the risk of false positives increases (2). This means that the probability that a person with a positive result (SARS-CoV-2 detected) is actually infected with SARS-CoV-2 decreases as the prevalence decreases, regardless of the claimed specificity.”
This explains that the likelihood of a person being positive for COVID-19 is less when COVID-19 is less common in the population. Writing this in an ad is probably a reminder to examine positive cases in areas with a low number of patients with COVID-19 and not a general statement that PCR is inaccurate.
None of these passages “admits that the PCR test at high amplification rates alters the predictive value of the tests and results in a large number of false positives”, as some have suggested. False positives in PCR tests are extremely uncommon, with the number currently ranging from 0.8% to 4.3%, according to the UK government. The vast majority of false positives occur as a result of analytical errors, having no relation to the PCR test or cycle number.
As for the rumor that WHO has reduced the number of PCR cycles in its guidelines, this seems unfounded and all that is recommended is that the cycle numbers be adjusted according to the test manufacturer’s instructions. The WHO suggests that in the case of borderline testing, in which the patient has a very low viral load, a second test should be performed.