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Monday, August 31, 2020
By Klaus Wedekind
“Test, test, test” worked well at the start of the corona pandemic in Germany. But now capabilities are running out, time and money are wasted on tests that are too expensive and time consuming, and many people have to be quarantined for no reason. It is time to adjust the strategy.
To allow for a daily life at half normal in the corona pandemic, many tests must be performed, especially in a preventive way. However, the PCR tests used to date have proven quite inadequate for this: not only are they too complex and expensive, they are also too accurate for mass use. Therefore, scientists call for a change in strategy.
Depends on viral load
Virologists recently shared in rare drive Hendrik streeck and SPD health expert Karl Lauterbach an article in the New York Times. It’s about the fact that a large number of people in the US test positive even though they are probably not contagious at all. Because with the standard PCR test there are basically only two possible results: yes or no. In other words: the test is really only there to detect the virus. A positive result says nothing about whether a patient is, was or will be sick. And you don’t know if it’s contagious either.
That’s not enough, says epidemiologist Michael Mina of Harvard’s TH Chan School of Public Health. The amount of virus in a patient’s body is crucial to knowing whether or not it is contagious. It is irresponsible for this fact to be neglected. German experts also call for a change in strategy. Charité virologist Christian Drosten is one of them. “Tests for infectivity are needed rather than infection,” he wrote in “Die Zeit.” Common PCR tests already provided the necessary information on viral load. “If we were to rely on ourselves to derive a tolerance threshold for viral load from the scientific data now available, medical officials could immediately discharge those whose viral load has already fallen below the breakdown period threshold. It would probably be the vast majority, “Drosten said.
Limit value for PCR tests too high
The information that matters is the Ct value. It corresponds to the number of PCR cycles that are necessary until the virus genome (RNA) is positively signaled. Therefore, a higher value speaks of a lower infectivity. A corresponding amount of live virus is deduced from the amount of RNA. The Robert Koch Institute (RKI) has determined a value greater than 30 as a criterion by which patients can come out of isolation as a guidance aid for physicians “on the basis of previous experience”. This is justified by the fact that a corresponding amount of virus is not sufficient to multiply in the laboratory.
The “New York Times” writes that most PCR tests already give a positive result with a Ct value below 40. This corresponds to an article in the “Pharmazeutische Zeitung” according to common practice in Germany. But “any test with a cycle threshold above 35 is too sensitive,” Juliet Morrison, a virologist at the University of California, told the New York Times. A reasonable limit would be between 30 and 35. Her colleague Michael Mina is in favor of the Ct value recommended by the RKI.
Dramatic effects
A lower cutoff value would have a dramatic effect on the recorded number of infections. In a New York lab in July, 794 tests came back positive, of which half would have been lost if the Ct value had been lowered to 35, writes the New York Times. With a threshold of 30, the tests would only have reached 30 percent. In Massachusetts, that number would have been 85 to 90 percent negative instead of positive, Mina says. Certainly the numbers in Germany would seem less dramatic, incorrectly being compared time and again to those of the spring, when only people with symptoms were tested.
However, one problem with this is that people who were recently infected may also have low concentrations of the virus. This objection generally applies to corona tests, so a second test is generally necessary to be safe in asymptomatic people. This, in turn, increases the costs and effort of already expensive and time-consuming PCR tests.
Inaccurate rapid tests to perfection
Therefore, Mina and other American scientists advocate the use of rapid tests. Also in Germany, more and more experts, for example Karl Lauterbach and the virologist Alexander Kekulé, are asking for this. Methods that have already been approved abroad or are about to be available in Germany include antigen tests that detect virus proteins rather than genetic material. The simplest are as simple as pregnancy tests.
Rapid tests are less accurate than PCR tests, but this is even an advantage when testing asymptomatic people. Because they don’t work if the viral load is too low to be contagious. Therefore, rapid tests are probably the right tool to use for preventive testing and mass testing rather than wasting PCR. Michael Mina says that perhaps not all carriers would be caught. But rapid tests would surely find the most contagious people, including super spreaders. “That alone would reduce epidemics to practically zero.”
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