Recently , some sites and Internet users have been reporting that the World Health Organization ( WHO ) has " accept the turn of COVID-19 cocksure cases is grossly overestimated " , and that the " new " PCR trial run is invalid . The reports appear to stem from anannouncementby the WHO advise lab technicians and testing scientists to cautiously validate the PCR results , and to ensure that each test is cautiously considered . Written in quite technical speech communication – it was purpose at PCR scientist and expert in the line of business – it is intelligible how many misconceive the wording , but what is said in the announcement is not necessarily what has reached public ears .
PCR Testing
Firstly , let ’s explain what a PCR test is . Polymerase Chain Reaction , or PCR , is simply a method acting for duplicating the genetical stuff in a sample to make more of it , so that further test can be performed . The psychometric test hertz through different stages that are repeated up to 50 times , each one repeat the DNA / RNA exponentially – the less genetic stuff in the sample , the more wheel needed to get it to a high enough concentration .
Scientists usually execute PCR when they have a sample of DNA or RNA and they are looking for specific genes within that sample but the amount of genetic material is too pocket-sized . Rummaging through a flyspeck DNA sampling for a specific chronological succession without PCR would be like essay to playWhere ’s Waldowith a really blurry effigy – you need to make it clear 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 notice the presence of SARS - CoV-2 in a patient , the examination take a swab from the throat , which will contain virus speck if the person is infected . A especial form of PCR , forebode reverse transcriptase PCR ( rt - PCR ) , is then used to amplify the amount of genetic material in the sampling before it is tested for the bearing of specific computer virus gene .
WHO discombobulation
PCR tests have been in force play to turn back for COVID-19 transmission for almost a year now , and have been our skillful method for realise the preponderance of the computer virus . However , recently the WHO released a report relating to how the test outcome should be cautiously scrutinise to control its truth , and some have contain to understand this as the psychometric test being inaccurate .
This is the first of two musical passage written by theWHO :
“ WHO guidance symptomatic testing for SARS - CoV-2 submit that heedful interpretation of weak positive results is needed ( 1 ) . The cycle per second threshold ( Ct ) need to detect computer virus is reciprocally proportional to the patient ’s viral freight . Where test results do not correspond with the clinical presentment , a new specimen should be taken and retested using the same or dissimilar NAT engineering . ”
This handing over simply calls for any positive trial run result that give a weak sign to be scrutinise carefully to check the result is a true positive and not a pretended positive , and if the patient role has symptoms of COVID-19 , repeat the tryout to be exactly sure of the result . Likely written as a reminder or an update to their official protocol , this procedure is already done in many COVID-19 testing location and is by no means state the PCR test is inaccurate .
It also talk about thecycle doorstep ( Ct ) , which is the number of PCR cycles needed to make the signal strong enough to register . Some people lay claim that a mellow bit of oscillation will give a overconfident solution for anything – this is not straight , more cycles are needed when the genetic material assiduity is very low , as it is in COVID-19 testing . More cycles can not highlight the presence of something that is n’t there .
The second passage posit this :
“ WHO prompt IVD users that disease prevalence alters the predictive value of test result ; as disease prevalence decrease , the risk of false convinced increases ( 2 ) . This means that the probability that a somebody who has a positive outcome ( SARS - CoV-2 discover ) is unfeignedly infected with SARS - CoV-2 decreases as prevalence reduction , no matter of the claimed specificity . ”
This explains that the likeliness a person is positive for COVID-19 is lower when COVID-19 is less common in the population . Writing this into an announcement is likely a monitor to scrutinize plus shell in areas with a downhearted routine of COVID-19 patients and not a blanket program line that PCR is inaccurate .
Neither of these passages " admits that PCR testing at high amplification rate alter the predictive value of the test and results in a vast figure of false positive degree " as some have paint a picture . fake positive in PCR tests are extremely uncommon , with the number presently range from 0.8 percent to 4.3 percent according to theUK government . The huge majority of false positives occur as aresult of analytical errors , having no coitus to the PCR test or Hz identification number .
As for the rumor that the WHO has cut the number of PCR cycles in its guidelines , this appears unwarranted and all that is urge is that cycle numbers are adjust in proceed with test manufacturer instructions . The WHO intimate that in case of a mete test , in which the patient has a very small viral load , a 2d run should be carried out .
