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In Germany, many people have to be quarantined even though they have so few virus fragments that they cannot infect anyone. After investigation of WDR, Ed and SZ, many health authorities do not receive adequate laboratory values.
By Markus Grill, NDR / WDR and Mara Leurs, WDR
The test, which tests people for the coronavirus nationwide, has a great advantage, which is also a disadvantage: This PCR test is very sensitive. This means that if the sample is taken correctly, it will hardly leave an infected person undetected. But the high sensitivity also means that many people test positive, even though the infection is so advanced in them that they have hardly any virus in their bodies, and they can no longer infect anyone else.
The so-called Ct value gives an indication of the amount of virus a patient is carrying. Shows how many rounds the PCR must run before the virus genome is detected. In a patient with a large amount of viral material in their body, the test often stops after 10 to 15 rounds of CT scans, lab doctors say. However, if the PCR takes more than 30 rounds to detect viral material, it is very likely that a patient is no longer contagious. According to the Robert Koch Institute website, no virus can be reproduced in laboratory tests from human samples with a Ct value greater than 30.
But many labs that evaluate PCR tests do not stop analysis at a Ct value of 30, but usually only at 37 or 40, as Uwe Dittmer explains. The deputy director of the German Society for Virology heads the department of virology at Essen University Hospital. Since many suspected cases with symptoms of the disease are analyzed there, the Ct value is “in most cases well below 30”. However, if you evaluate large numbers of generally asymptomatic people, “then many Ct values will certainly rise to a range above 30 as well.”
Many are no longer contagious
An article in the “New York Times” caused a stir this week, reporting that test data from Nevada, Massachusetts, and New York suggest that up to 90 percent of PCR tests show Ct values so high that patients barely still had virus. Therefore, epidemiologist Michael Mina of Harvard University advocates setting the Ct limit at 30.
Berlin crown expert Christian Drosten was covered in his latest podcast Ed–Info also with this question. “A Ct of 30 in one laboratory is not the same in terms of viral load as a Ct of 30 in another laboratory,” he warned. In principle, however, it was considered correct to quantify the viral load of patients, for example, by recording the Ct value of a reference sample. “I don’t think it’s wrong now when people say in the US, Let’s set a Ct value, I would accept it,” Drosten said.
Result of “thinning” of saliva
However, Essen virologist Dittmer doesn’t really trust the findings from the US. He suspects that the many high Ct values reported there are due to the fact that the samples are taken differently than those from Germany. In the US, for example, samples are often not taken in the deep throat, but only with saliva, which presumably dilutes the result. He also doubts that the smear is always taken correctly from volunteers in Germany conducting tests on motorways, for example.
Dittmer’s lab performs PCR testing not only for the Essen clinics, but also for the entire city. As a general rule, do not communicate the Ct value to the health authorities. “That is not planned. Usually we just say if someone is positive or negative.”
The doctor. Upon request, Kramer and his colleagues will inform you that the Ct value will be used internally to evaluate the sample, but will not be passed on to the health department. Of the 963 positive samples since the end of July, almost every second would have had a Ct value of 30 or more, says laboratory physician Jan Kramer, who is a board member of the association “Accredited medical laboratories.” But the high number is also because it includes follow-up tests and follow-up checks, and the virus in infected people is declining every day after a spike.
Too little information reduces the chance of discovering super spreaders
A survey of Ed, WDR and SZ among health authorities has shown that value is often not passed on at all. However, this means that health authorities without a Ct value generally have no evidence of how infectious a person is who has tested positive. This also means, for example, that they cannot target potential super-spreaders whose PCR test indicates a very high viral load.
The Cologne health department announced that the Ct value was assigned “only in individual cases according to medical indication.” Dortmund and Leipzig respond to the request that they do not obtain any Ct value from the laboratories. In Lübeck, the office does not receive any Ct values either, but often receives additional information on whether the test was positive or only slightly positive. Munich health department responds, some labs conveyed the value, but others did not. Wiesbaden, on the other hand, receives “more and more laboratory results with a Ct value”, and in Karlsruhe the value is reported “mainly”.
In Schwerin, the value is used for quarantine decisions.
The Schwerin health department is already taking the Ct value as an occasion for a quarantine decision. In consultation with the lab doctor, the office “released two patients from quarantine” due to low Ct values, as the city spokeswoman said upon request.
Virologist Dittmer also questions that people with a Ct value of more than 30 have to be quarantined. However, a decision in this regard cannot depend solely on the Ct value. First, you need to ensure that the sample has been taken correctly, and second, you need to clarify whether the patient who tested positive is in the phase of a growing or decreasing infection. However, this last question can only be answered by performing a second test shortly thereafter in patients with a Ct value greater than 30.
RKI: value is just an “analytical detail”
Even the Robert Koch Institute (RKI) cannot answer whether and how many health authorities in Germany experience Ct values from laboratories. Upon request, the RKI only reports: “We assume the labs will clarify the further procedure with the health department if the results are questionable.” After all, according to the RKI, the Ct value is “an analytical detail that supports the interpretation of the test result.” However, value is only one factor in the evaluation. A Ct value greater than 30 can be “used as a criterion” when released from quarantine, according to the RKI.
