January 17, 2021

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Block sampling would help with shortage of reagents for COVID-19 testing

A technique of handling viral tests known as “pooling” or block sampling could be the key to managing the increase in cases and tests that laboratories are handling amid the rebound of the pandemic of the COVID-19 coronavirus.

This technique, in addition, would help to deal with the shortage of reagents and also reduce the delivery time of results.

As detailed by Dr. Marcos López, manager of the Trust of Public Health of Puerto Rico, the reference laboratories of the Island "have the best equipment available in the market" and a qualified personnel that can carry out the processing of tests under this technique, and in fact one of the laboratories, Core Plus is already doing it.

“The laboratories are working at full steam. But many samples are coming. And although the kits to work them are also coming, there are delayed tests, and more are coming, "explained the doctor, recalling that there are tests that are done with priority, for example, in hospitalized cases awaiting a transplant.

He also clarified that, regarding the reagents, the shortage is due in part to the fact that the companies that supply them are doing so according to the number of cases reported, and how Puerto Rico has had its setbacks in the manner of report their figures, and also was reporting fewer cases, because the amount of reagents they have sent is not necessarily according to reality .

“And now we have to do more tests, because we have many cases. So we have to be smarter when assigning tests. If you were exposed, then go home first, isolate yourself, and wait two or three days to allow time for the incubation to have already occurred and that test to be more effective, "he said, before explaining the bulk sampling technique in detail, or added, as it is also known.

As explained the COVID-19 testing process has two steps . In the first, a process is carried out to extract all the genetic virus material in the sample, which comes out as RNA (deoxyribonucleic acid). In the second part, in that genetic material, a specific signature is sought, which is the genetic material of the Covid-19 virus (SARS-CoV-2).

To explain it in a way that can be understood more easily. It is like when cars pass through a toll, and you look for those of a particular manufacturer and model, but then in the second step you identify one that has a specific board, which separates it from the rest.

López assured that it is a process "very sensitive" that allows you to find those specific brands of COVID-19 that do not vary, and that can do so even if the amount of virus is very small.

And here is where the technique comes into play of block sampling to speed things up, also taking into account that "the prevalence of the virus is still relatively low, because we are not at the level of New York or Florida, and I know that many samples will be negative."

" So I take five samples, I extract the viral genetic material, and instead of analyzing it separately, since I know that I have a very low prevalence and a high possibility that they will be negative, in the second step I will look for the virus in all five at the same time. "Explained the doctor. "If it doesn't come out, well, all five are negative. If it comes out, well, I go back, and I specifically check the tests separately to know what the virus has. ”

And putting this on a larger scale, in the case, for example, that a laboratory that receives all at once 450 samples, “I can do about 90 samples per kit more or less. So you would need 5 kits for the 450 samples. And each process lasts on average about six to eight hours . So you can go two days running to make those samples because the technologists also have to rest. "

" And now what I do is that, with 'pooling', I have the five samples together. So I'm going to create 90 samples representing those 450, and I can do it with one kit. And obviously since the test is so sensitive I can assume that the 5 samples in that group are all negative, "he continued. “And if any test is positive, let's say 7 wells, because I separate those 35 individual samples that set me off the alarm, and I analyze them separately. But I have already discarded the other 415 samples. ”

“ That is, in 12 hours I can analyze all those 450 samples, with only two kits, and I save 60% time and reagents. And the times to report the result, and also the costs, drop a lot, "said the doctor. In other words, if there is the capacity to process 1,500 tests per day, with 'pooling' you will be able to do 4,500 and you will gain a lot of time. You can increase the capacity by three times. ”

López added that the technique is endorsed by the Federal Food and Drug Administration (FDA, in English) and that it is a process that is done routinely with the handling of blood products and derivatives to detect the presence of viruses, without a single transfusion infection being reported in more than 30 years.

However, the doctor specified that the technique could only be used if the level of positivity remains low “because when it reaches 10% positivity it no longer works for you.”

“So it is very important that all of us do our part to stop the virus. You can have all the tests in the world, but if people don't do their part, it won't work, "insisted the doctor.

" If everyone, with good judgment, for two months, wears the masks correctly, we can stop the contagion. And we already saw it here in Puerto Rico with the initial closing. But then everything suddenly opened and all that we had achieved was lost. But we already did. We already saw that it can. So you have to educate people. It is not that it does not come out, it is that you put on the mask and use it well. And so we can achieve it, "he urged.

He added that it is also important to maintain frequent hand washing and social distancing measures . “For example, those politicians with those campaigns, these crowds of people, all of this has to be stopped. We must assess the evidence that we have available, "he insisted.

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