Πώς δουλεύουν τα τεστ για τον κορωνοϊό

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pipinos1976
Δημοσιεύσεις: 1194
Εγγραφή: 08 Απρ 2011, 23:52

Πώς δουλεύουν τα τεστ για τον κορωνοϊό

Δημοσίευση από pipinos1976 »

Μία σύντομη δημοσίευση (στα αγγλικά) για το πώς ανιχνεύεται η ύπαρξη κορωνοϊού σε έναν ανθρώπινο οργανισμό.

How Do Coronavirus Tests Work?
How Do Coronavirus Tests Work?

A deluge of COVID-19 testing options will soon flood the market. Here’s how they work

Months into the COVID-19 pandemic, the United States has finally moved from relying entirely on a single, flawed diagnostic test to having what may soon be an onslaught of testing options available from private entities. The U.S. Food and Drug Administration over the last three weeks has authorized the emergency use of more than 20 diagnostic tests for the novel coronavirus known as SARS-CoV-2.

Those add to the hundreds of tests that researchers are developing globally. The Foundation for Innovative New Diagnostics (FIND) in Geneva, Switzerland keeps a running global list of COVID-19 tests that, as of 3 April, neared 400.

To help our readers sort through the deluge of diagnostics, here, we provide a simple explainer on coronavirus testing.


What kinds of tests are used to confirm COVID-19?

Diagnostics for COVID-19 typically fall into two broad categories: tests that detect proteins associated with the virus, known as immunoassays, and tests that detect the virus’s genetic code, known as nucleic acid or molecular tests.

Nucleic acid-based tests are the most sensitive in early detection of infection, and have been widely used during the COVID-19 pandemic. These tests typically rely on a decades-old technique called reverse transcription polymerase chain reaction, or RT-PCR.

First a sample is taken by swabbing the nasal passages or throat. To find evidence of the virus, researchers use PCR to copy and amplify any segments of viral genetic code found in the sample, which makes it easier to detect. This typically involves adding reagents and enzymes, and raising and lowering the temperature of the mixture, known as thermal cycling.

Software then determines when the number of copies of the target sequence exceeds a threshold, indicating if the novel coronavirus is present, and at what concentration. (Scientists have known and tracked changes in the virus’s genetic code for several months. This information has enabled scientists to develop the necessary components for PCR analysis.)

PCR is the gold standard in viral diagnostics. “Because the sensitivity is so good, it should be the best mechanism for picking up copies of the virus at very early stages of infections,” says Paul Yager, a bioengineer focused on the development of diagnostics at the University of Washington in Seattle. “Even if someone is asymptomatic—not yet showing fever or cough—you might very well still pick up a positive.”

Diagnostic developers and labs around the world have customized their PCR tests to detect the novel coronavirus. Many of the available thermal cycler machines reside in large centralized laboratories that accept samples by mail. Results can take days due to the logistics of getting the sample to the lab.

Some hospitals and other health care settings, and research labs, have machines on site capable of PCR analysis. These devices can provide results in a couple of hours or less. The newest nucleic acid tests can produce results in minutes. Some of these are being customized to detect SARS-CoV-2.

For example, health giant Abbott has a device called ID NOW, introduced in 2014, that’s the size of a toaster and provides flu, strep, and respiratory syncytial virus test results in minutes. Last month, the company quickly put together an assay for COVID-19 testing that is compatible with its ID NOW platform. Positive results can be ready in five minutes, according to the company.

Abbott on 27 March announced that it had received for this device an Emergency Use Authorization from the FDA—a green light to market the test during the current public health emergency. The company will be making the tests available this week to urgent care settings in the United States, where the majority of ID NOW instruments are in use, the company says.


What are immunoassays?

The other primary way to test for COVID-19 is with an immunoassay. Rather than analyzing genetic material, these tests detect proteins produced either by the virus itself, or by the body’s immune system in response to the virus. The tests can detect evidence of past or current infections.

Viruses are covered in protein structures, called spikes, that are unique to that virus. As the body’s immune system kicks into gear to fight the infection, it produces its own unique proteins, called antibodies, that attach to the protein structures on the virus.

The unique linkage between antibodies and their respective viral proteins, or antigens, makes both parties good targets for diagnostics. Immunoassays take advantage of this. Such tests use antibodies and antigens as bait to capture their respective partners.

A sample—typically blood, plasma, or serum—is run through the test, usually in the format of a lateral flow assay. Antibodies in the blood sample will recognize and bind with the antigens on the test. Likewise, viral antigens in the blood sample will bind with the antibodies on the test. Any binding that takes place indicates a positive result.

Immunoassays aren’t as sensitive as nucleic acid tests in the earliest days of an infection. “The challenge is you need an awful lot of virus to see a signal,” says Yager. But immunoassays tend to be easier to use, portable, and fast—important features for use in a busy emergency department. Rapid tests are particularly useful as triage tools, in that they can quickly tell health care workers whether a patient should go to the quarantined part of the hospital, or the regular part of the hospital.

Some types of immunoassays, called serological assays, can be used weeks after an infection to confirm that a person had the virus and recovered. These are in development for coronavirus. For example, the University of Minnesota and the Mayo Clinic say they are readying such a test, which could be helpful in determining which health care workers have immunity.

So far, all of the COVID-19 diagnostics that have received the FDA’s Emergency Use Authorization have been nucleic acid-based tests. But many immunoassays are in the pipeline. Nearly half of the tests on FIND’s global list of COVID-19 tests in development are immunoassays.

What does not exist is an at-home, sensitive, disposable test for COVID-19, similar to a pregnancy test. “There’s virtually nothing” like that for COVID-19, says Yager. In fact, the FDA on 20 March warned of “unauthorized, fraudulent test kits that are being marketed to test for COVID-19 in the home.”

The FDA’s warning, along with updates to its policy, stymied development of at-home sample collection kits as well. These kits provide sample collection equipment so that people can swab themselves at home and send the sample into laboratories for analysis. Several companies developing such kits reported putting that work on hold, after the warning and further guidance from the FDA.
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pipinos1976
Δημοσιεύσεις: 1194
Εγγραφή: 08 Απρ 2011, 23:52

Re: Πώς δουλεύουν τα τεστ για τον κορωνοϊό

Δημοσίευση από pipinos1976 »

Ένα άλλο τελείως περίεργο περιστατικό - ή μάλλον σειρά γεγονότων - συμβαίνουν αυτή την στιγμή στην Αφρική. Στην Τανζανία, όπως ισχυρίζεται ο Πρόεδρος της χώρας, τα εργαστήρια βγάζουν αποτελέσματα εκτός τόπου και χρόνου. Μάλιστα, έστειλε, όπως έχει ισχυριστεί, δείγματα από διάφορα φυτά και ζώα και έχει λάβει θετικά αποτελέσματα για μερικά από αυτά, όπως από μία κατσίκα και από ένα φρούτο παπάγιας. Από έγκυρους οργανισμούς στην Αφρική (το παράρτημα του CDC - Centres for Disease Control and Prevention για την Αφρική) δηλώνεται ότι τα τεστ δουλεύουν κανονικά, πράγμα που περιπλέκει ακόμη περισσότερο τα πράγματα. Μέσα σε όλα αυτά βάζουμε και την κόντρα του κυβερνητικού συνασπισμού στην Τανζανία και η εξήγηση του τί έχει συμβεί γίνεται ακόμη πιο δύσκολη.

Ενδιαφέρον θα έχει να δούμε αν όντως έχουν γίνει αυτά που περιγράφει ο Πρόεδρος της χώρας, οπότε το επόμενο βήμα θα πρέπει να είναι η συζήτηση για το αν έχουν γίνει έλεγχοι ή ο υπεύθυνος έβγαζε τυχαία αποτελέσματα, αν όντως έχουν γίνει έλεγχοι, αν αυτοί έχουν γίνει σωστά, και, αν έχουν γίνει σωστά, μήπως η ακρίβεια των ελέγχων αυτών δεν είναι η απαιτούμενη. Ή αν συμβαίνει κάτι άλλο. Πολλά τα ζητήματα και πρέπει να ελεγχθούν 1-1 οι παράγοντες για να βρεθεί η λύση. Προς το παρόν εμείς κρατάμε μια επιφύλαξη για την περίπτωση να υπάρχουν tests με μειωμένη ακρίβεια και υψηλά ποσοστά σφάλματος.

Οι ειδήσεις αυτές προέρχονται από το BBC και μπορεί να βρεθούν στην διεύθυνση: https://www.bbc.com/news/topics/cjnwl8q4qdrt/tanzania
Tanzanian coronavirus laboratory boss suspended

Sammy Awami
BBC News, Dar es Salaam
15:37, 4 May


The Director of Tanzania’s National Health Laboratory and its quality manager have both been suspended pending investigation.

The suspension of the top officials comes a day after President John Magufuli questioned the credibility of coronavirus testing kits being used by the lab.

In a live broadcast from his hometown of Chato in north-western Tanzania yesterday, Mr Magufuli revealed that he secretly commissioned testing of randomly collected non-human samples, which were assigned names and ages.

He said some samples taken from animals and fruits returned inconclusive results, some testing negative and others showing positive results.

“Vehicle oil for instance, labelled Jabir Hamza aged 30-years-old [male], tested negative. We sent a jackfruit sample which we named Sara Samweli, a 45-year-old female - the results were inconclusive. When we sent a papaya [paw-paw] sample and named it Elizabeth Anne, aged 26 years, that papaya was positive,” said Mr Magufuli.

He added that samples from a bird and a goat both tested positive, while one from a rabbit was found to be indeterminate.

A statement from the health ministry said the minister has formed a committee of 10 people to investigate the conduct of the laboratory and submit the results by 13 May.

Testing for coronavirus will still be conducted at the laboratory.

Tanzania’s approach to fighting coronavirus has been criticised as being not strict enough and dominated by secrecy.

The government has refuted the allegations saying it carefully follows the guidelines issued by the World Health Organization.

According to the most recent health ministry figures, which were published last week, the country has 480 confirmed cases of coronavirus, including 167 recoveries and 16 deaths.
Magufuli wrong on faulty coronavirus tests - AU

15:03, 7 May

The head of the African Union (AU) body that is co-ordinating the continent's coronavirus response has rejected the suggestion from Tanzania's president that tests for the virus are faulty.

Last weekend, President John Magufuli, who has been criticised for his approach in dealing with the virus, said that he had secretly had some animals and fruits tested at the laboratory and that a paw-paw, a quail and a goat had returned positive samples.

But responding to a question about Mr Magufuli's comments, John Nkengasong, head of the Africa Centres for Disease Control and Prevention (Africa CDC), told journalists that the tests that Tanzania and other African countries are using had "been validated and we know that they are performing very well".

He went on to say that co-operation, co-ordination and collaboration were vital if the fight against Covid-19 was to be successful.

"No country in Africa is an island" when it comes to dealing with the virus, Dr Nkengasong said.

What is needed is a coherent message right from the leadership of the continent down to the local level, he added.

In Tanzania, the government's main priority appears to be keeping the economy going as President Magufuli refuses to countenance the closure of markets or the lucrative port in the commercial hub of Dar es Salaam.

Mr Magufuli has insisted that people should still go to places of worship to find "true healing" - despite these being areas where social distancing is difficult to follow.
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