Most positive COVID-19 PCR test results are true positives; WHO information notice didn’t change threshold or criteria for a positive COVID-19 test
The COVID-19 PCR test detects the presence of the genetic material of the virus that causes COVID-19, SARS-CoV-2. The test is highly sensitive, meaning that it can detect very small amounts of the virus in infected individuals, including those in the early or late stages of infection. The test is also highly specific for SARS-CoV-2, meaning that it doesn’t detect the presence of other viruses, such as other members of the coronavirus family that cause the common cold. Although no tests are 100% accurate, given the PCR test’s high sensitivity and specificity, most positive COVID-19 PCR test results are true positives.
Inaccurate: The WHO Information Notice is a reminder to laboratory personnel to interpret PCR test results with care under certain conditions, such as when a patient’s symptoms are inconsistent with the test result. The Notice didn’t “admit” the test generates many false-positive results. It also didn’t lower the threshold for a positive test or change the criteria for a positive test result across the board.
Claims aimed at discrediting the COVID-19 PCR test and, by extension, public health responses to the pandemic, have repeatedly appeared throughout the pandemic. An information notice regarding COVID-19 PCR testing, published by the World Health Organization (WHO) on 20 January 2021, became the new basis for such claims.
These claims were published in outlets such as Zero Hedge, Children’s Health Defense, LifeSiteNews, and the Epoch Times, which previously published health misinformation about COVID-19 (see examples here, here, here, and here). The claims also went viral on Twitter, and tweets captured in screenshots also went viral on Facebook (see here and here).
These articles and tweets interpreted the notice in various inaccurate and misleading ways. For example, some claimed that the notice shows the WHO “admits” the PCR tests produced many false-positive results, therefore the number of COVID-19 cases reported by public health authorities is inflated. Others claimed that it introduced a change in PCR methodology which would reduce the number of COVID-19 cases. Still others claimed that the WHO changed the criteria for a positive test result. These articles and posts received more than 39,000 interactions and more than 14,000 shares on Facebook, according to the social media analytics tool CrowdTangle.
These claims are inaccurate. Indeed, reading the information notice itself makes it clear that the WHO didn’t “admit” that the COVID-19 PCR test generates a large number of false-positive results, didn’t change the PCR methodology for “thresholds”, and didn’t change the criteria for positive test results across the board.
In short, the notice doesn’t mention any of the purported claims these articles and posts attributed to it, as Gideon Meyerowitz-Katz, an epidemiologist at the University of Wollongong, pointed out in his response to an inaccurate tweet. The inaccurate tweet has since been deleted but is archived here:
Amazing that someone can tweet something like this, which is both absolute nonsense and NOT EVEN RELATED TO THE SCREENSHOT, and it still goes viral
The endless bullshit of COVID-19 denial https://t.co/QeM9c2OwTP
— Health Nerd (@GidMK) January 21, 2021
“We still have 190,000+ cases per day that are diagnosed. Let’s say that 1% of those are false positives (which could happen if you move the Ct by a marginal amount), that would only reduce our DAILY cases by 1,900 – so, [not that] much.”
Although disease prevalence can increase the likelihood of false-positive COVID-19 test results, as the notice stated, Smith pointed out:
“Disease prevalence (this is cumulative cases; not just one-day totals) can increase the risk of false positives IF prevalence goes way down. We are a LONG way away from that happening. Prevalence will need to be very, very low for this to be the case.”
The COVID-19 PCR test detects the presence of the virus that causes the disease by amplifying a small part of the virus’ genetic material. The number of amplification cycles needed to arrive at a threshold considered to be “positive” is also called the cycle threshold (Ct) value. The Ct value is dependent on the amount of virus in a sample. The more virus present, the fewer amplification cycles are needed to reach the positive threshold, while a low viral load requires more amplification cycles to reach that threshold.
Ian Mackay, a virologist and associate professor at the University of Queensland, explained in this blog post that the information notice was simply a reminder to laboratory staff to “Read the instructions and understand the purpose of testing”. He lamented that “this lab-[focused] advice has been taken by those with malicious intent, or with too little understanding of the topic, and [blown up] into something else entirely wrong.”
Mackay highlighted a series of examples showing how the notice was spun in various ways to become the basis of misinformation on COVID-19 PCR testing, such as this (now-deleted) tweet by urologist David Samadi:
“The misunderstanding […] is around the WHO’s comment not to mess with the threshold for determining the threshold cycle (CT), if that contravenes the real-time RT-PCR test kit’s instructions for use (IFU) and you have no experience with doing so.
I have no idea how that was misinterpreted into becoming about changing the number of cycles used [in] the RT-rPCR though. This bold but wrong statement was quickly retweeted (now deleted thankfully) and spread far and wide (the account has 330,768 followers and a television audience).”
With regards to the claim that the notice recommends that the threshold for a positive result be lowered:
“[The] tweeter seems to have fallen into the same trap of misunderstanding, misreading or not reading at all the WHO Information Notice. They seem to think it says that the cycle number could be reduced. The notice doesn’t say this.”
And about the misleading claim that the WHO changed the criteria for a COVID-19 positive test result:
“Context is really lacking here. The WHO Notice doesn’t state this as written and this will likely lead some who only read headlines to walk away with the wrong impression.
[…] The Notice discusses weak positive results (late or high CTs) – not all positives. It goes on to say that if the test doesn’t fit with the patient’s health (I’d add contact and epidemiological history) status, a new sample should be collected and the test repeated. A lab may use the initial test or a different test (good labs are armed with tests that target more than one genetic region of SARS-CoV-2 or come from more than one commercial supplier).”
A commonly repeated but inaccurate claim about the COVID-19 PCR test is that it generates many false-positive test results. Some conclude that this means the number of COVID-19 cases reported by public health agencies is inflated. However, as Meyerowitz-Katz explained in this article, “Most Positive Coronavirus Tests Are True Positives”:
“While there were some early validations that showed slightly higher numbers of false positives, we’ve now got enormous samples run by researchers from across the world that show the rate of false positives in PCR tests for COVID-19 to be around 1 in 1,000 or lower.
In other words, it is incredibly rare for a COVID-19 test that comes back positive to be a false positive. Almost unheard of. It happens, but very rarely, particularly because many tests are confirmed, i.e. run twice.”
This New York Times article, which dealt with the implications of high Ct positive test results, was misinterpreted by several outlets to mean that 90% of test results in the U.S. are false positives due to the PCR test’s sensitivity. As Health Feedback explained in this review, this interpretation fails to distinguish between the PCR test’s ability to confirm an infection—which is what case numbers measure—with the test’s ability to determine contagiousness.
This misinterpretation also uses the term “false positive” inaccurately to describe people with a high Ct positive result. A false-positive result indicates that a person tested positive but does not have the disease. This is not the case for people with a high Ct value positive result. Regardless of whether the test had a high or low Ct value, a person is or has been infected if they test positive. This also means that it is correct to consider a person with a positive result and high Ct value as a COVID-19 case.
Overall, the claim that a WHO Information Notice “admits” that the PCR test produces many false-positive results, or that it changed the threshold or criteria for a positive test result, is false. Reading the notice makes it clear that it doesn’t state any of the above.
As explained in our review, positive COVID-19 test results generated by PCR testing are largely true positives. And regardless of whether the Ct value is high or low, a positive test indicates that the person is or has been infected with the virus, which qualifies them as a COVID-19 case. COVID-19 cases reported by public health authorities are therefore not an inflation due to “false positives”, but are genuine cases of infection.
NOTE (6 Feb. 2020):
After our review was published, Breitbart corrected their article by removing the inaccurate claim that the WHO had changed the COVID-19 PCR testing criteria and the misleading claim about false-positive test results (see archive of corrected article).
- 1 – Lalkhen et al. (2008) Clinical tests: sensitivity and specificity. Continuing Education in Anaesthesia Critical Care & Pain.