PCR tests are useless if you’re asymptomatic…

…and other aspects of PCR tests

It should be clear that I am not in accord with the ideological aspects of the content of this (most obviously, his apparent belief, confounded by over 100 years of the history of journalism,  that the media should somehow hold the powers-that-be to account as if the media was not a significant arm of the state).

Deepl translation from here

Prof. Dr. Martin Zizi, former President of the Ethics Committee and of the Commission for Medical Ethics within the Belgian Department of Defence, in charge of the relations with the Order of Physicians between 1997 and 2004, former Scientific Director and Head of the Division of Epidemiology and Biostatistics, researcher in Molecular Biology and Biophysics; he was an advisor/expert for the Belgian authorities, the EU and the UN.

This article is a response to the publication “Do PCR tests overestimate Covid-19 cases? – published in the new “fact checking” section of La Libre Belgique.

In a Democracy, the press should control the power in place and not the citizens. The solution: a real testing policy without exclusivity and based on reality, not on a “rubber band” that keeps us all in fear and allows them to decide anything.

Recently, an article mentioned me in a fact-checking column (The Source, a new column in La Libre) about PCRs and their misuse throughout this SARS2 crisis. I find the intention commendable and I share the desire of the Editor-in-Chief and the journalist. However, on reading the article, I believe that the result is not nearly informative enough, the conclusion being to reassure on the use of PCR and to contrast it with other types of tests. As announced in a previous article, we need to work on solutions, and this light fact-check publication gives me the opportunity not only to explain the problem to the general public, but also to outline the solution to this problem. So I have to come back to this topic because it is far too important to be diluted even with brilliance. Note that I have been in the press twice on this subject, and I have never said or written that RCPs have no role to play as the article describes ((1), (2), as well as many exchanges between La Libre and myself). I also make the difference between symptomatic and asymptomatic every time.

What is the problem with the PCR tests that have been made the cornerstone of this health, social and economic debacle? In fact, there is not one, but two perfectly distinct problems and they must be clearly distinguished.

Problem #1. PCR ≠ Infections.

This means that a positive PCR does not automatically equate to an infection (a given for all molecular biologists). Germs simply live among us. For example, almost all of us have staphylococcus germs on our skin at one time or another. The frequency is between 20 and 30% at any one time(3), and 66% of people have this germ on their skin intermittently, but repeatedly(4). If we did PCRs on thousands of people tomorrow, we would almost always have between 30% and 66% of the ‘positive cases’ in the whole random population. How many of us have a staph infection of the skin? Almost none! Do you understand the problem now? So a disease is not the same as a positive PCR test. On the other hand, if we have a skin disease, then PCR can help the clinician to show that it is staphylococcus, to know if it is resistant or not, and to know if it is another germ, and in any case, will help the doctor to prescribe the appropriate treatment. The context of the test is therefore absolutely cardinal.

To be sick with a virus, measuring a dozen or millions of viruses per measurement – and PCR does this – means nothing if you don’t understand the notion of the infection threshold. Indeed, each virus has a different threshold for making us ill; for hepatitis B this threshold is very low, but for HIV it is higher. For SARS2 we need about one million particles per millilitre in our bronchial tubes to become infected and ill(5) (references even quoted by Sciensano).

As a reminder, PCRs amplify the genetic material to be measured per cycle, each cycle doubling the mass of what is to be measured. But as with any instrument, whether in the scientific field or elsewhere (music, construction, etc.), you have to start by calibrating the tool to be used. For the PCR, this means detecting a series of viral solutions with a known number of particles (10 viruses/ml, 100 viruses/ml, 1000 viruses per ml, and so on). It is then possible to match these different virus concentrations to PCR multiplication cycles: 2, 3, 4 … 20, 30, 40 cycles. This makes it possible to define the number of cycles necessary (the Ct or Cycle Treshold) to reach the famous threshold of one million viruses per ml (below which there is no infection). It is important that this calibration is done for all laboratories, as there are differences between the machines. It should be noted, however, that after a very large number of cycles, any instrument will inevitably fall outside the PCR range. The WHO initially published standardised protocols at 35 cycles – and more recently has indicated in an addendum that each laboratory should indeed calibrate its machines (reference given in the insert). And in Belgium? We were happily doing 35 cycles, whereas the million per ml is reached at around 23 cycles according to our own standards (this figure fluctuating only by a few units depending on the machine used)!

It should also be remembered that PCR is a backwards test – the longer you run the reaction, the less you measure what you are looking for. 23 cycles give us the threshold of one million viruses per millilitre – which is necessary to say that we have a risk of infection – which is still not an infection because each of us can be more or less sensitive to the virus (another real problem but one that is outside the scope of this article). 33 cycles means that we measure 1000 times less virus (because 2 exponent 10 = 1024, i.e. the difference in cycles between 33 and 23), so instead of a million viruses, the sample contains only 1000! Above this threshold (called “Ct” in PCR results) of 23, one can conclude that: This is not an infection! If these tests are repeated with too many cycles, the results become random and non-specific and are no longer reliable: i.e. the SAME sample could be positive once, negative once… so the test gives no valid information.

Figure 1.

The problem is that PCRs are considered positive by Sciensano from 100 copies onwards, which is much lower than the threshold needed to be counted as a case (an infected patient). As for the conclusions on contagiousness, the term “potential” does not help. A Doctor receiving these answers cannot interpret them correctly as he is not a PCR specialist – but he knows whether his patient has symptoms or not, and THAT should be his criterion.

Can a NON-INFECTED patient be contagious? Of course not. That would be an absolute NO-SENSE. The last column (on the right) is what I explain when I say that PCR ≠ Infection.

Above 25, it is obvious that the person is NOT infected – so cannot be a “case” – based on these erroneous PCR’s, most governments have gone off the deep end… alas, the cost to our societies is enormous.

Table showing the number of viral copies in a calibration, the number of cycles for a gene X, the Sciensano and RAG recommendations and above all the reality of an infection according to the threshold.

(RAG – Risk assessment group)


Let’s go even further: if I explain to you that in ICUs (Intensive Care Units) in Belgium, some of these patients who have not contracted COVID are nevertheless labelled “COVID” because the test is “positive”. And this even in the absence of any clinical picture of respiratory infection sometimes! This cannot happen in our good kingdom? Don’t believe me, talk to the ICU nurses and doctors. So the problem of the PCRs even extends in part to the hospital beds…

And when you know that in France and Germany, PCRs are done at 38 or even more than 40 cycles, in Ireland at 45 cycles (some protocols and standards were shared so comparisons are possible), you can only see that the problem goes beyond our borders… Are you beginning to understand why this debate on PCR cycles and calibration is essential?

Problem #2. PCR ≠ Contagiousness.

A positive PCR does not equate to a contagious person. It is the ‘tail’ of infections. The Source article does a better job on this – and I thank the journalists for their informative clarity, as they are not scientists.

Fig. 2: If we test NON-symptomatic people, we are therefore six times more likely to find a positive but non-contagious PCR test than a positive and contagious one. Even if we take a safety margin of a factor of two (the possible contagion period is eight days), we are still four times more likely to have a positive but non-contagious PCR test. In this case, we can say that only 25% of the tests correctly indicate a risk of contagion.


The SARS-CoV 2 virus remains in our body for weeks after the disease is over – so we are no longer contagious at that time. There is a lot of literature on this (this point is no longer contentious at all). What the article does not highlight is that this non-contagious period is 4-6 times longer than the contagious period! If the “window” for concluding that someone is “dangerous to others” is a few days, then the chance of getting it wrong – i.e. having a positive test while being non-contagious – is obviously much greater.

Furthermore, the fact-checking does not mention that I provided them with the Lancet study that was used to introduce the subject(6), as well as another article(7) in the New England Journal of Medicine that covers this subject and tries to explain how to do it better – to make intelligent use of tests. Which is my aim too. The Source article mentions a scientific reference which, after studying and explaining this problem, concludes that between 50% and 75% of PCR tests are false positives for this ‘infection tail’ reason, but points out – without any data at all – that the 75% figure is surely wrong because, and I quote, “We don’t randomly test […] people suspected of having COVID because they have symptoms or their contacts”. This quote is completely false, as we shall see later. Did the scientist questioned by the journalist from La Libre Belgique present a table with statistics? On what basis are the symptoms estimated? After clinical medical consultation or after a declaration on honour – as is allowed? Did the journalist only check the number of cycles carried out in the Belgian labs?

With the necessary hindsight, it is difficult to estimate the proportion of correct tests compared to incorrect ones, as this would have required a systematic correlation between PCR, symptoms and serological tests [which are tests that measure antibodies in the blood of people who are actually infected] – something that was apparently not done. So we can only have an estimate based on current scientific knowledge. But if the residence period of the virus in the body is 4 to 6 times shorter than the period of contagion, we could deduce that a significant proportion of these tests do not reflect a risk of contagion at all. It is therefore high time to stop the war of figures on this subject – especially when people with no symptoms are being tested en masse – and admit that we don’t know… but then why present these tests as the only possibility of measurement? This raises questions.

There is a third problem with these PCR tests: the huge financial stakes.

This was one of the issues that the journalists told me they were going to address in this fact-check and that they don’t even touch on in their opus. Why not? It would be good if the potential conflicts of interest of some of the advisers on this issue were investigated. Search the various university websites for start-ups or companies that provide this expensive service (test statistics are published on the website of La Libre Belgique) and check for possible links with experts or advisers. Checking shareholders’ agreements. At a rate of 600-2000 tests per day at peak for a small lab [data independently checked by telephone], and at a price of 47 Euros, this is a lot of money. How much is it? And for a large university lab or private companies? How much? We would have to analyse of course, but it would not be surprising to reach figures of several hundred million euros just for small Belgium and for PCRs. All this money for tests that help us so little and allow us to justify this medical, social and economic suicide in an ad hoc manner?

Moreover, you probably don’t know this… but we have been through this before. After 9/11, my phone was constantly clogged with companies wanting to “help me deal with the crisis” and so the pressure to recommend purchases of PCR machines (between 80 and 100 machines) and to push a vaccine (yikes!) against anthrax was enormous. And I never recommended PCRs (nor this vaccine) in my capacity as an advisor to the Defence Cabinet but also to the Prime Minister’s Office and to Health (via the Intercab). Measuring anthrax by PCR was expensive and unnecessary because anthrax lives among us. As experts – and especially during crises – the commercial pressures and temptations are enormous…

There is also a side problem: false oppositions. There are those who oppose PCR testing to antigenic and serological testing – for reasons of conflict of interest, and they hide it well. And I fear that the press does not understand that they are being taken for a ride. My communications do not attack PCR and do not want to promote other types of tests. I have nothing to sell, neither test nor drug, and especially not – unlike some of our experts and journalists – wind – and I am out of those biotechnology activities that made my daily life for nearly 40 years! I simply explain, in full agreement with the WHO, that PCR is a powerful tool for diagnostic confirmation if you are ill with symptoms. But – as the WHO points out – we have to be very careful in our conclusions if we test people who are not sick or without symptoms. I say this less politely than the WHO, I agree. It is a necessary alarm – not a whisper of discomfort! I have personally written to the press, and explained to the journalist that a testing strategy is needed and I invite readers of all my LinkedIn posts to check for themselves by giving them keywords to search. I don’t give an opinion, but try to elevate the debate.

About the other tests – and this makes me sad because they would save lives – it would be good if others would talk about them. Again, the Source article does not reflect our conversation. I never told the reporter that these antigen tests were all calibrated for this purpose. On the contrary, I explained to the journalist that these tests could be perfectly calibrated to be positive(8) only during the period when the person tested is contagious. This is a problem of the mass of chemical reagents to be put in the test kits. So I deplore the “drowning of fish” with these rates of true or false positives and comparisons made on tests that were never optimised for this period of contagion.

Finally, if we know that a test gives erroneous results for multiple reasons ((9) and (10)) and was imposed in a financial blur(11), and serves to block any debate and break everything(12), it is not only legitimate, but even our duty to say so. It is a bit of a stretch to say that the PCR, despite its limitations, is the only screening option in view of all that has been decided and the NON-COVID deaths it has caused. Let’s call it for what it is: rubber-band testing, which justifies everything. This debate around CRP is TOO important to ignore, because it is the basis on which lockdowns, green or red zones, or travel control are decided… It is the same basis on which COVID beds in ICU (intensive care units) are calculated.

It should be noted at this point that The Source, in addition to the problems mentioned above, contradicts itself. Indeed, it reports through Dr. L. Cornelissen that Cornelissen that “since non-symptomatic people are not tested, there are no problems”. This is false information because asymptomatic people are tested in Belgium and in large numbers. We have been testing them since the beginning, and it was publicly stated by the authorities that we would stop these tests during the holidays in November 2020. The tests were officially resumed at the end of November. La Libre Belgique as well as other newspapers) announced these important official decisions with important articles [see Edition of Oct. 19, 2020, title: Backtracking: people without symptoms will no longer be tested]. The resumption on 23 November was announced everywhere [see RTBF website of 14 Nov]. In this article, there is mention of asymptomatic people who have had contact and I quote [“And then, that we can follow up contacts and trace the chains of asymptomatic people”]. Chains of asymptomatic people! This is not a sexually transmitted disease, but a zoonosis! So tracing does not give a correct picture of the spread of this virus (and outdoor contact will be safe compared to contact in a closed environment) but let’s move on.

Moreover, when the PCR tests were resumed on a large scale [see La Libre Édition of 25 Nov. 2020], Commissioner Corona himself said: “[…] the rapid tests (he is talking about PCR) are reliable in people who remain asymptomatic with a high viral load and who are therefore contagious” (sic). What is the proportion of these people? In general, non-symptomatic people have low or zero viral loads(13). Moreover, travellers (the vast majority of whom are symptomless) and even the parents of children who have been in contact with a case are tested all the time, so one patient plus two other people are tested. Moreover, when anyone can ask for a test after signing a Declaration on honour that he/she has the symptoms of COVID, it becomes surreal. As the fact-checking journalist confuses the problem of the threshold of detection (problem #1) and the problem of the persistence (presence) of the virus in our airways long after there is no longer a risk of contagion (problem #2). Confusing these two different problems is precisely why PCRs are misused.

The Source also mentions another issue that I must elaborate on: asymptomatic people

I quote The Source: “An April 2020 study in Nature magazine even estimates that 44% of infections in households occur during the pre-contamination phase, before the first symptoms appear. A trend followed by the WHO, which states that “it is mainly just before infected people develop symptoms (i.e. two days before) and at the very beginning of the disease that they are most contagious”. I’m going to surprise you, but I completely agree. Indeed, the vast majority of contaminations take place within family bubbles and closed environments and not outside.

We are dealing with potential pre-symptoms and PCR has its place. However, we should stop talking nonsense and making amalgams. The majority of people with COVID who are asymptomatic are in the rest of the population, not around the patients. There have been many studies on this subject which have not been able to show a risk of contagiousness. One of the largest studies ever carried out was in Wuhan on nearly 11 million people(14). It shows – contrary to previous studies – that these asymptomatic people – even if they are PCR positive – emit very little virus (which is logical because they are not ill and therefore do not cough!) and that their rate of contagiousness is almost zero. Why does this go unnoticed?

The problems with PCRs that I explain in this article are not new and there are examples of PCRs failing(15). The press (this does not concern La Libre belgique) reported that it was fake news so here is another much needed fact-check. In British Columbia, there was a pseudo-epidemic of SARS1 in 2003 measured by supposedly perfect PCR tests. In the end, this “outbreak” – which killed eight people, six of whom died of bacterial pneumonia – was due to another perfectly banal and benign corona. For the record, there are seven human coronaviruses (four that cause the common cold as well as SARS1, MERS, and SARS2). At the time, officials had the presence of mind to test for antibodies, thus avoiding panic and fear. In 2006 in New Hampshire (USA), an outbreak of pertussis (B. Pertussis) turned out to be a creation of PCRs. This false alarm problem is well known and was discussed in the Lancet in 2006(16).

If journalists were doing their job as inquiring minds, they would not have to phone and drink in the incorrect or outright false words of some of my ex-colleagues. All they would have to do is read Sciensano’s notes – available online. It is amazing that Sciensano writes one thing and does the opposite. Whether it is a lie or stupidity, the question remains for me, and I hope that many citizens will make the effort to ask themselves. This can be checked by anyone, does not require specialised knowledge and reveals the following facts:

On page 10 of their SARS2 fact sheet, Sciensano writes in black and white that the “Viral RNA ≠ infection”. I quote: “A test-based strategy is hindered by known prolonged shedding of viral RNA, which does not equate with infectiousness”. The quote is given in full at reference(17).

Contact tracing was reported to return positive in only 1% of cases. 22 patients among the 2761 contacts linked to 100 cases demonstrated. Who are we kidding here? A 1% chance of being sick if you are a contact?

And the surprises don’t stop there. Sciensano mentions in his notes on PCR(19) that :

Hamster infections correlate with cell culture infections but not with PCRs – and that doesn’t shock anyone? Hamster or human, this demonstrates the limitations of correlation conclusions via PCR.

In France, it was shown that PCRs above a Ct of 34 are not indicative of infection. “Patients with samples with Ct values ≥34 did not excrete infectious viral particles.” So why does the RAG consider them to be cases? This begs the question.

A Canadian study, tells us that if Ct > 24 on human samples, these samples were not infectious. Quote (given in the appendix in full) “Cell culture growth was significantly reduced when RT-PCR Ct values >24 (primers targeting the E gene)

In Germany, the threshold of infection was shown to be above 1 million viruses per ml in human bronchial sputum samples. “German group concluded, based on the viral loads of nine hospitalized patients, that little risk of infectivity remained below a viral load of 100,000 viral RNA copies per ml sputum.

Who are we kidding… On the one hand, Sciensano puts the right references with correct information on their website, and on the other hand, they completely ignore it and create and maintain a climate of panic fear to the decision makers and all the populations of the EU based on measurements not correctly interpreted. I don’t know about you, dear readers, but I am deeply shocked by all this, and isn’t it time that the Press finally did its job: Control the Power, read, understand, educate itself in order to be able to inform.

What is the solution?

There should be a ‘testing path’ – an algorithm. I will limit myself to the basics – a real protocol/path should/will be established by experts and colleagues:

1. See patients in person, based on symptoms and clinic – and confirm the potential diagnosis of COVID by PCR. For this purpose PCR will be highly effective.

2. For contacts – these are either pre-symptomatic if they eventually get sick, or those who will remain perfectly asymptomatic – focus mainly on contacts in closed environments where contamination occurs (the famous bubbles, public transport, centrally ventilated buildings… which are the best foci for dispersal of any virus). The others are not. The mania for doing everything all the time is deleterious.

3. For the non-symptomatic – the entire population – PCR not being a suitable tool – antigenic tests perfectly calibrated to make the positive result coincide with the period of contagion. The false negative rate is defined by the chemical design of such tests – so it could be perfectly reliable. If there is any doubt, one should go to one’s family doctor and a PCR may follow. I also did not say that these tests were already calibrated, but that they should be and that it would not take much time. I put a relevant reference and invite you to have a good look at the figure in the paper below. Only a test that is less sensitive than PCR could effectively match the period of contagion and identify the contagious.

4. Serological tests (which measure antibodies in the blood) should also be used. Because after more than 17 months, it is time to do a randomised serological study corresponding to our population – as any crisis requires – because it will be the only reliable method to estimate the proportion of people infected but who remain carriers. – This would allow us to calculate a true IFR (Infection fatality rate). This would break the cycle of fear, and reassure the population instead of the CFR (case fatality rate) – which only measures our inability to manage COVID cases in a timely manner. This was the subject of numerous reports, including one written by some forty people under the leadership of the Hoover Institute at Stanford(20). (20) Moreover, this report was given by me to La Libre on two occasions. I do not believe that it is intellectually honest or a service to the public to present myself as a detractor of PCR, having practised it continuously between 1993 and 2014, partly on environmental germs… Why? The question is asked.



2. https://www.levif.be/actualite/international/covid-19-et-strategie-de-depistage-mensonges-ou-stupidite/article-opinion-1403483.html?cookie_check=1617737036

3.Current Epidemiology, Etiology, and Burden of Acute Skin Infections in the United States. Khaye et al. Clinical Infectious Diseases, Volume 68, Issue Supplement_3, 1 April 2019, Pages S193–S199, https://doi.org/10.1093/cid/ciz002

4.Does the nose know ? An update on MRSA decolonization strategies 
Abad et al. Curr Infect Dis Rep 2013, 15:455– 64.

5. Some sources concerning the viral load required to penetrate cells and used for extrapolation of a contamination threshold:
– Wölfel R. et al. Virological assessment of hospitalized patients with COVID-2019. 
Nature. 2020 May;581(7809):465-469. doi: 10.1038/s41586-020-2196-x. Epub 2020 Apr 1. Erratum in: Nature. 2020 Dec;588(7839):E35. PMID : 32235945
– Qiu X, et al. Defining the role of asymptomatic and pre-symptomatic SARS-CoV-2 transmission, a living systematic review. 2021 Jan 20. Clin Microbiol Infect. 2021;S1198-743X(21)00038-0. doi:10.1016/j.cmi.2021.01.011- La Scola B et al. Viral RNA load as determined by cell culture as a management tool for discharge of SARS-CoV-2 patients from infectious disease wards. 
Eur J Clin Microbiol Infect Dis. 2020 Jun;39(6):1059-1061. doi: 10.1007/s10096-020-03913-9.
– RAG interpretation and reporting of SARS COV-2 PCR results (Sciensano) https://covid-19.sciensano.be/sites/default/files/Covid19/20201208_Advice%20RAG%20Interpretation%20and%20reporting%20of%20COVID%20PCR%20results.pdf

6. Lancet article reporting on the work of a group of OFFICIAL scientists to evaluate the various tests. They attempt to clarify the place of tests – The key phrase – see Figure 2: The short window of transmissibility contrasts with a median 22–33 days of PCR positivity (longer with severe infections and somewhat shorter among asymptomatic individuals). This suggests that 50–75 % of the time an individual is PCR positive, they are likely to be post-infectious.” Mina et al.

7. Article in the New England Journal of Medicine, originally from Harvard School of Public Health – which explains that the CONTEXT of a test is important and explains that a LESS sensitive test can be calibrated to target the period of contagion only. – Rethinking COVID-19, a strategy for containment. Mina et al. https://www.nejm.org/doi/pdf/10.1056/NEJMp2025631?articleTools=true

8. Idem.

9. https://www.kairospresse.be/a-propos-des-tests-par-pcr-lettre-ouverte-a-mes-collegues-qui-conseillent-nos-gouvernements/

10. https://www.levif.be/actualite/international/covid-19-et-strategie-de-depistage-mensonges-ou-stupidite/article-opinion-1403483.html?cookie_check=1617737036

11. Current Epidemiology, Etiology, and Burden of Acute Skin Infections in the United States. Khaye et al. Clinical Infectious Diseases, Volume 68, Issue Supplement_3, 1 April 2019, Pages S193–S199, https://doi.org/10.1093/cid/ciz002

12. Does the nose know ? An update on MRSA decolonization strategies 
Abad et al. Curr Infect Dis Rep 2013, 15:455– 64.

13.  Old article in the Libre – the same questions arose about white powder envelopes after 9/11.  https://www.lalibre.be/debats/opinions/comment-vivre-avec-le-biologique-51b87dd2e4b0de6db9a89871

14. Study on PCR-positive asymptomatics in Wuhan Cao et al, dans la revue Nature https://www.nature.com/articles/s41467-020-19802-w

15. 3 references about pseudo-epidemics that turned out to be artefacts of PCR testing British Columbia in 2003. 
Outbreaks of Respiratory Illness Mistakenly Attributed to Pertussis — New Hampshire, Massachusetts, and Tennessee, 2004-2006
 – Alerte du Lancet à ce sujet :

16. Idem.

17. From Sciensano, lire en p.10 de leur fact-sheet – 2 citations qui contredisent toute la communication officielle !
A test-based strategy is hindered by known prolonged shedding of viral RNA, which does not equate with infectiousness. Assesment of viral load might help in these cases but viral loads are usually semi-quantitatively expressed as cycle threshold-values, which differ according to technical lab circumstances and the gene target(s). (NB. Le verbe au conditionnel ‘might’ est utilisé)
Whilst viral culture studies are difficult to interpret and all studies have important methodological limitations, the contact tracing study of Chen et al (Taiwan) is of high quality. In the study, 100 confirmed cases (of which 6 severe) and their 2,761 close contacts are followed up. Only 22 secondary cases occurred. No secondary cases were observed in those exposed to the index case more than 5 days after onset of symptoms (SAR 22/1,818 = 1.0 % [0.6%-1.6%] first 5d vs. 0/852 = 0 % [0-0.4%]) (150).

18. Idem

19. From Sciensano, lire en p. 5 – explication sur les interpretations des tests PCR, notes collationnés par le Risk Assessment Group (RAG) 
A Chinese study using a hamster model found that transmission of SARS-CoV-2 correlated well with detection of infectious SARS-CoV-2 from respiratory tract samples using in vitro Vero cell cultures, while not with detection of viral RNA.
A French study including 155 patients (183 samples) observed a strong correlation between Ct value of RT-PCR with primers targeting the E gene (samples of upper and lower respiratory tract) and sample infectivity in a cell culture model. Patients with samples with Ct values ≥34 did not excrete infectious viral particles.
A Canadian study looking at 90 RT-PCR SARS-CoV-2 positive samples (endotracheal and nasopharyngeal) found that cell culture growth was significantly reduced when RT-PCR Ct values >24 (primers targeting the E gene). In a multivariable model they found that, for every 1 unit increase in Ct, the odds ratio for infectivity decreased by 32 %.
A German group concluded, based on the viral loads of nine hospitalized patients, that little risk of infectivity remained below a viral load of 100,000 viral RNA copies per ml sputum. They also estimated the RNA concentration for <5 % isolation success to be 5.40 log10 RNA copies per ml (95 % confidence interval −4.11–6.51) (upper and lower respiratory tract). The primers used in this study targeted the E and RdPr genes.

20. Hoover Report – Preparing for the Next Pandemics. 
A collective work, the tables on pages 7 and 45 (tests) are relevant to this article. Dr. M. Zizi was one of the 40 scientists invited to write and review this book. https://www.hoover.org/sites/default/files/research/docs/mcmaster_webready_revised.pdf