Rapid antigen test methods

A positive virus test indicates a current infection, while a positive antibody test indicates a previous infection.Other techniques include CT scans,checking for elevated body temperature, checking for low blood oxygen levels,and detection by trained dogs.

Detection of the virus

Detection of viruses is usually done by looking for the virus' internal RNA or protein fragments on the outside of the virus.Tests that look for viral antigens (parts of the virus) are called antigen tests.

There are various types of tests that look for viruses by detecting the presence of viral RNA.These are called nucleic acid or molecular tests,after molecular biology.As of 2021,the most common form of molecular testing is reverse transcription polymerase chain reaction (RT-PCR) testing.Other methods used in molecular testing include CRISPR, isothermal nucleic acid amplification,digital polymerase chain reaction,microarray analysis,and next-generation sequencing. 

Reverse transcription polymerase chain reaction (RT-PCR) test 

Polymerase chain reaction (PCR) is a process that amplifies (replicates) a small, well-defined fragment of DNA hundreds of thousands of times,producing enough fragments for analysis.The test samples are treated with certain chemicals, allowing DNA to be extracted.Reverse transcription converts RNA to DNA.

Reverse transcription polymerase chain reaction (RT-PCR) first uses reverse transcription to obtain DNA,which is then amplified by PCR to generate enough DNA for analysis.Therefore, RT-PCR can detect SARS-CoV-2 that only contains RNA.The RT-PCR process usually takes several hours.These tests are also called molecular or genetic analyses.Real-time PCR (qPCR) offers advantages such as automation, higher throughput, and more reliable instrumentation. It has become the preferred method.This combined technique is described as real-time RT-PCR or quantitative RT-PCR, sometimes abbreviated as qRT-PCR rRT-PCR or RT-qPCR, although RT-PCR is sometimes used -PCR or PCR.Minimum information to publish guidelines for quantitative real-time PCR experiments (MIQE) proposed the term RT-qPCR,but not all authors adhere to this.

The average sensitivity of rapid molecular tests depends on the brand. For ID NOW, the average sensitivity was 73.0%, and the average specificity was 99.7%; for Xpert XPress, the average sensitivity was 100%, and the average specificity was 97.2%.

In diagnostic testing, sensitivity is the measure of the test's ability to identify true positives, and specificity is the measure of the test's ability to identify true negatives.As with all tests, including diagnostics and screening, there is usually a trade-off between sensitivity and specificity, so higher sensitivity will mean lower specificity and vice versa.A 90% specificity test will correctly identify 90% of uninfected people, leaving 10% of false positive results.Samples can be obtained by a variety of methods, including nasopharyngeal swabs, sputum (expected material),throat swabs,[36] deep airway material collected through a suction catheter, or saliva.Drosten et al. Commenting that for SARS in 2003, "From a diagnostic point of view, it is important to note that nasal and throat swabs do not appear to be suitable for diagnosis, as these materials contain much less viral RNA than sputum, and if Only these, the virus may escape the detection material tested.The sensitivity of clinical samples by RT-PCR was 63% for nasal swabs, 32% for throat swabs, 48% for stool, 72-75% for sputum, and 93-95% for bronchoalveolar lavage. The likelihood of detecting the virus depends on the method of collection and the time elapsed since infection. According to Drosten, tests with throat swabs are only reliable for the first week. The virus may then leave the throat and multiply in the lungs. During the second week, sputum or deep airway collection is preferred.

Saliva collection may be as effective as nasal and throat swabs,although this is uncertain.Saliva sampling can reduce risk to healthcare professionals by eliminating close physical interaction.It is also more comfortable for the patient.People in quarantine can collect their own samples.The diagnostic value of saliva testing depends on the sample site (deep throat, oral cavity, or salivary glands).Some studies have found that saliva has higher sensitivity and consistency compared with swab samples.On August 15, 2020, the U.S. FDA granted emergency use authorization to a saliva test developed by Yale University that gives results within hours.

On January 4, 2021, the US FDA issued a warning about the risk of false results from the Curative SARS-Cov-2 Assay real-time RT-PCR test, especially false negative results.

Viral loads measured in upper respiratory specimens decreased after symptom onset.After recovery, viral RNA was no longer detectable in upper respiratory tract specimens from many patients.In those who did, RNA concentrations three days after recovery were often below the range for reliable isolation of replication-competent virus.There was no clear correlation between the length of illness and the duration of viral RNA shedding after recovery in upper respiratory specimens.

Types of Antibody Tests

Rapid Diagnostic Test (RDT)

RDTs typically use small, portable, positive/negative lateral flow assays that can be performed at the point of care. RDTs can process blood samples, saliva samples, or nasal swabs. RDT produces colored lines to indicate positive or negative results.Enzyme-linked immunosorbent assay (ELISA) ,ELISA can be qualitative or quantitative and usually requires a laboratory.These tests usually use whole blood, plasma, or serum samples.Plates are coated with viral proteins, such as the SARS-CoV-2 spike protein.The sample is incubated with the protein,allowing any antibodies to bind to it. Antibody-protein complexes can then be detected with another antibody wash that produces a color/fluorescence readout.

Neutralization test

Neutralization assays assess whether a sample antibody prevents viral infection in test cells.These tests sample blood, plasma, or serum.The test grows cells that allow the virus to multiply (for example, Vero E6 cells). By varying the antibody concentration, the researchers could visualize and quantify how much of the test antibody was blocking viral replication.

Chemiluminescent Immunoassay

Chemiluminescent immunoassays are quantitative laboratory tests.They take samples of blood, plasma, or serum.The sample is mixed with known viral proteins,buffer reagents,and specific enzyme-labeled antibodies.The result is luminous.Chemiluminescent microparticle immunoassays use magnetic, protein-coated microparticles.Antibodies react with viral proteins to form complexes.Enzyme-conjugated secondary antibodies are added and bound to these complexes.The resulting chemical reaction produces light.The radioactivity was used to calculate the number of antibodies.The test can identify several types of antibodies,including IgG, IgM, and IgA.

Other tests

Sniff test

Sudden loss of smell can be used to screen people for COVID-19 on a daily basis.A study by the National Institutes of Health showed that people infected with SARS-CoV-2 were unable to smell a 25% mixture of ethanol and water.Because of the various conditions that can cause anosmia,the sniff test is not definitive, but suggests the need for a PCR test.Because anosmia precedes other symptoms, extensive sniff testing is called for.Sniff tests are often ignored by health care bureaucracies,despite the fact that they are quick, easy, and capable of self-administration on a daily basis.This has led some medical journals to write editorials supporting the use of the sniff test.

Imaging

Typical visible features on CT initially include bilateral multilobar ground-glass opacities with peripheral or posterior distribution.Identified COVID-19 more accurately using CT compared to RT-PCR.As the disease progresses,subpleural dominance,mad paving, and consolidation may occur.Chest CT scans and chest X-rays are not recommended for diagnosis of COVID-19.Radiological findings of COVID-19 lack specificity.Chest x-rays, computed tomography scans, and ultrasounds are all ways to detect coronavirus disease.