COVID-19 testing

COVID-19 testing involves analyzing samples to assess the present or past presence of SARS-CoV-2.

 the 2 main types of tests detect either the presence of the virus or antibodies produced in response to infection. Molecular tests for viral presence through its molecular components are wont to diagnose individual cases and to allow public health authorities to trace and contain outbreaks. Antibody tests instead show whether someone once had the disease. they're less useful for diagnosing current infections because antibodies may not develop for weeks after infection. it's used to assess disease prevalence, which aids the estimation of the infection deathrate . This variation has likely significantly impacted reported statistics, including case and test numbers, case fatality rates and case demographics. Because SARS-CoV-2 transmission occurs days after exposure, there's an urgent need for frequent surveillance and rapid availability of results.

Test analysis is usually performed in automated, high-throughput, medical laboratories by medical laboratory scientists. Rapid self-tests and point-of-care testing also are available and can offer a faster and less expensive method to test for the virus although with a lower accuracy.

Methods 

Positive viral tests indicate a current infection, while positive antibody tests indicate a previous infection. Other techniques include a CT scan, checking for elevated blood heat , checking for low blood oxygen level, and detection by trained dogs.

Detection of the virus 

Detection of the virus is typically done either by looking for the virus's inner RNA, or pieces of protein on the surface of the virus. Tests that search for the viral antigens are called antigen tests.

There are multiple sorts of tests that look for the virus by detecting the presence of the virus's RNA. These are called macromolecule or molecular tests, after biology . the foremost common form of molecular test is the reverse transcription polymerase chain reaction test. Other methods utilized in molecular tests include CRISPR, isothermal macromolecule amplification, digital polymerase chain reaction, microarray analysis, and next-generation sequencing. RT-PCR can thereby detect SARS-CoV-2, which contains only RNA. The RT-PCR process generally requires some hours. These tests also are referred to as molecular or genetic assays.

The combined technique has been described as real-time RT-PCR or quantitative RT-PCR and is usually abbreviated qRT-PCR, rRT-PCR or RT-qPCR, although sometimes RT-PCR or PCR are used. The Minimum Information for Publication of Quantitative Real-Time PCR Experiments guidelines propose the term RT-qPCR, but not all authors adhere to the present .

Average sensitivity for rapid molecular tests depend upon the brand. For ID NOW, the typical sensitivity was 73.0% with a mean specificity of 99.7%; for Xpert Xpress the typical sensitivity was 100% with an average specificity of 97.2%.

In a diagnostic test, sensitivity may be a measure of how well a test can identify true positives and specificity is a measure of how well a test can identify true negatives. For all testing, both diagnostic and screening, there's usually a trade-off between sensitivity and specificity, such higher sensitivities will mean lower specificities and vice versa. A 90% specific test will correctly identify 90% of these who are uninfected, leaving 10% with a false positive result.

Samples are often obtained by various methods, including a nasopharyngeal swab, sputum, throat swabs, Drosten et al. remarked that for 2003 SARS, "from a diagnostic point of view, it's important to note that nasal and throat swabs seem less suitable for diagnosis, since these materials contain considerably less viral RNA than sputum, and therefore the virus may escape detection if only these materials are tested."

Sensitivity of clinical samples by RT-PCR is 63% for nasal swab, 32% for pharyngeal swab, 48% for feces, 72–75% for sputum, and 93–95% for bronchoalveolar lavage.

The likelihood of detecting the virus depends on collection method and the way much time has passed since infection. consistent with Drosten tests performed with throat swabs are reliable only in the first week. Thereafter the virus may abandon the throat and multiply within the lungs. within the second week, sputum or deep airways collection is preferred.

Collecting saliva could also be as effective as nasal and throat swabs, although this is often not certain. it's also more comfortable for the patient. Quarantined people can collect their own samples.

On 15 August 2020, the US FDA granted an emergency use authorization for a saliva test developed at Yale University that gives results in hours.

On 4 January 2021, the US FDA issued an alert about the danger of false results, particularly false negative results, with the Curative SARS-Cov-2 Assay real-time RT-PCR test.

Viral burden measured in upper respiratory specimens declines after symptom onset. Following recovery, many patients not have detectable viral RNA in upper respiratory specimens. Among those that do, RNA concentrations three days following recovery are generally below the home in which replication-competent virus has been reliably isolated. No clear correlation has been described between length of illness and duration of post-recovery shedding of viral RNA in upper respiratory specimens.

Other molecular tests 

Isothermal macromolecule amplification tests also amplify the virus's genome. they're faster than PCR because they don't involve repeated heating and cooling cycles. These tests typically detect DNA using fluorescent tags, which are read out with specialized machines.

CRISPR gene editing technology was modified to perform the detection: if the CRISPR enzyme attaches to the sequence, it colors a paper strip. The researchers expect the resulting test to be cheap and straightforward to use in point-of-care settings. The test amplifies RNA directly, without the RNA-to-DNA conversion step of RT-PCR.

Antigen tests 

An antigen is that the part of a pathogen that elicits an immune response. Antigen tests search for antigen proteins from the viral surface. within the case of a coronavirus, these are usually proteins from the surface spikes. SARS-CoV-2 antigens are often detected before onset of COVID-19 symptoms with more rapid test results, but with less sensitivity than PCR tests for the virus.

COVID-19 rapid antigen tests are lateral flow immunoassays that detect the presence of a selected viral antigen, which indicates current virus infection . Antigen tests produce results quickly, and most are often used at the point-of-care or as self-tests. Self-tests are rapid tests which will be taken at home or anywhere, are easy to use, and produce rapid results. Antigen tests are often performed on nasopharyngeal, nasal swab, or saliva specimens.

Samples could also be collected via nasopharyngeal swab, a swab of the anterior nares, or from saliva. The sample is then exposed to paper strips containing artificial antibodies designed to bind to coronavirus antigens. Antigens bind to the strips and provides a visual readout. the method takes less than 30 minutes, can deliver results at point of care, and doesn't require expensive equipment or extensive training. this is often especially true for asymptomatic patients who have little if any nasal discharge. Viral proteins aren't amplified in an antigen test. A Cochrane review supported 64 studies investigating the efficacy of 16 different antigen tests determined that they correctly identified COVID-19 infection in an average of 72% of people with symptoms, compared to 58% of individuals without symptoms. Tests were most accurate when utilized in the first week after symptoms first developed, likely because people have the foremost virus in their system in the first days after they are infected. Routine antigen tests can quickly identify when asymptomatic people are contagious, while follow-up PCR are often used if confirmatory diagnosis is needed.

Antibody tests 

The body responds to a virus infection by producing antibodies that help neutralize the virus. Blood tests 

ELISAs are often qualitative or quantitative and generally require a lab. These tests usually use blood , plasma, or serum samples. A plate is coated with a viral protein, like a SARS-CoV-2 spike protein. Samples are incubated with the protein, allowing any antibodies to bind thereto . The antibody-protein complex can then be detected with another wash of antibodies that produce a color/fluorescent readout. A NAb is an antibody that neutralizes the infectivity of an epidemic particle by blocking its attachment to or entry into a susceptible cell; enveloped viruses, like e.g. SARS-CoV-2, are neutralized by the blocking of steps within the replicative cycle up to and including membrane fusion. Since most COVID-19 antibody tests return a positive result if they find only binding antibodies, these tests cannot indicate that the topic has generated protective NAbs that protect against re-infection. but this is often not established for COVID-19. A study of 175 recovered patients in China who experienced mild symptoms reported that 10 individuals had no detectable NAbs at discharge, or thereafter. How these patients recovered without the assistance of NAbs and whether they were at risk of re-infection was not addressed. and are gone after six years. Nevertheless, memory cells including Memory B cells and Memory T cells can last for much longer and may have the ability to reduce reinfection severity. Because various conditions can cause the loss of the sense of smell, a sniff test wouldn't be definitive but indicate the need for a PCR test. Because the loss of the sense of smell shows up before other symptoms, there has been a involve widespread sniff testing. Health care bureaucracies have generally ignored sniff tests although they are quick, easy and capable of being self-administered daily. This has led some medical journals to write down editorials supporting the adoption of sniff testing.

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