COVID-19 Rapid Antigen Tests (RATs) are lateral flow immunochromatographic assays that use antibodies to detect the presence of SARS-CoV-2, the virus responsible for COVID-19.
Rapid tests are easy to use with a short turnaround time (TAT) from sample collection to results. If performed within the infectious window, RATs are highly accurate and represent an effective method to screen a large number of people in a short time, and effectively manage the spreading of the virus.
The FDA has approved several different RATs; however, most of those only received FDA emergency use authorization.
What Are COVID-19 Ratios?
The CDC recommends calculating the COVID-19 ratio as the number of positive cases divided by the total number of tested patients. For example, if one patient testing negative for COVID-19 turns out to be positive, the COVID-19 ratio would be 1:1. If two patients testing negative for COVID-19 turn out to be positive, then the COVID-19 ratio would be 2:2.
This ratio helps identify hotspots and outbreaks so that public health officials can target their response accordingly for disease control purposes.
5 Facts About About COVID Rapid Antigen Testing
- Compared to a molecular test, a rapid antigen test have several advantages. They are cheaper and take less time. However, screening testing methodologies are less sensitive, and a confirmatory test result using a diagnostic molecular test may be required.
- RATs detect the presence of specific proteins of the SARS-CoV-2 virus.
- RATs can be performed by health practitioners and supervised trained professionals (point-of-care tests), or by non-trained individuals in a non-clinical setting (home-use tests).
- Rapid antigen tests are the most accurate if performed within 7 days from the appearance of symptoms.
- The average TAT of the COVID19 RATs is 10-15 minutes.
- Several other COVID-19 RATs are on the market, and many are currently undergoing clinical trials to assess their clinical performance required for approval from the public health authorities before being commercially available.
Application of COVID Rapid Antigen Testing
RATs can be used to diagnose acute infections or to confirm past exposure to the virus. They can also determine the prevalence of infection in a population. By identifying areas with high infection rates, public health officials can direct resources toward those locations.
The FDA has issued emergency use authorizations for COVID-19 RATs, allowing these tests for screening purposes. In addition, several states have implemented mandatory COVID-19 testing for all residents who are experiencing symptoms consistent with COVID-19.
These mandatory screening programs have been put in place to prevent the further spread of the disease.
Risks of the Procedure
There are no known risks associated with COVID-19 RAT testing. However, as with any laboratory procedure, these tests have some limitations, including:
- False-negative results – The test might give a negative result even if the person has contracted COVID-19. This might happen because the sample has not been collected appropriately or the viral load (the concentration of the virus in the nasal cavity) is still too low to be detected. Nevertheless, these events are rare, and the test is considered very accurate, and is a time-efficient screening method to help reduce the spreading of the virus.
- False-positive results – The test might give a positive result even if the person does not have COVID-19. This may happen if the test is taken too early or too late in the infectious stage (or infectious window).
- Incorrectly interpreted results – The interpretation of the result can be incorrect if the internal control of the test does not work as expected. In this case, the test needs to be repeated with a new collection kit.
Rapid Antigent Testing
COVID-19 lateral flow test is a rapid screening test that can deliver results within 15-30 minutes.
The way of working for lateral flow tests is somehow similar to the pregnancy tests. However, the sample for this test is usually collected from the throat or nose. Its kit contains viral proteins specific antibodies rather than pregnancy hormones.
Other than sending the test samples to laboratories (physically), lateral flow test kits are more convenient alternatives that require minimal training, are portable, and are small.
How do lateral flow tests work?
IIn the case of COVID-19, lateral flow tests analyze the samples or material collected from an individual’s throat or nose using a sterile swab.
Steps of lateral flow test:
- A cotton swab is inserted in both nasal passages and/or the throat and moved around in a circular motion approximately 5 times. A saliva sample may also be sufficient with some sample kits.
- After the sample has been collected, the nasal or throat swab is inserted into the collection tube and immersed in the liquid inside, moving it around to facilitate the release of the collected specimen within the tube.
- After it, a few drops of the liquid sample is deposited over a small absorbent pad contained in the specific testing kit. This is usually marked by the letter S (sample). The transfer is usually facilitated by the presence of an opening collection cap placed on top of the collection tube.
- This liquid is drawn along the pad by capillary action until it faces a strip, coated with antibodies specific for the SARS virus. If the virus is in the sample, it will bind the antibodies, producing a signal that is visible to the naked eye. These tests usually detect proteins of the virus’ nucleocapsid, which is the structure containing the viral genetic material. In the case of the SARS-CoV-2 virus the genetic material is made of RNA.
- A positive antigen test indicates the presence of a viral protein, and the rapid test will show up with a double-colored line pattern in correspondence to the letters T (test) and C (control). If you only have a coloured line in correspondence to the letter C, your test is negative, whereas if you see no lines or only one line on the T, the test is void and needs to be repeated.
- Results from RATs should be read no more than 5 minutes after the recommended read time.
How accurate are lateral flow tests?
According to the independent tests by FIND (Foundation for Innovative New Diagnostic), one of these lateral flow tests is clinically 76.6% sensitive and 99.3% specific, which makes them a little less sensitive than the PCR tests.
Molecular Testing (RT-PCR testing)
Molecular diagnostic tests are currently considered the gold standard. These tests detect the presence of the genetic material of the virus, also known as nucleic acid. The Reverse Transcriptase (RT) Polymerase Chain Reaction (PCR), or RT-PCR, is the most common nucleic acid test and is considered very specific and accurate, even in the presence of a low viral load.
RT-PCR tests can identify whether a person is infected with the SARS-CoV-2 virus, even before they start having symptoms.
Anterior nasal swabs are amongst the most common specimens collected for this specific test. If the SARS-CoV-2 virus is present in the patient’s sample, millions of copies of small virus segments are made, enhancing the signal.
RT-PCR can only be performed by trained laboratory scientists using specialized equipment in a laboratory setting. Given the nature of the test that requires the collection of the specimen, isolation of the genetic material, processing of the sample, and interpretation of the result, this type of test take longer (up to several hours) than RATs.
RT-PCR tests are considered as confirmatory tests, following an initial positive screening test.
How To Prepare The Patient for COVID Rapid Antigen Testing
Patients must understand that they must follow all precautions for COVID-19 testing.
Patients must understand that they must follow all precautions for COVID-19 testing. This includes carefully washing their hands and wearing a mask while awaiting the results.
Patients should be reminded that they must avoid spreading COVID-19 to others.
If they develop a fever or respiratory symptoms, they should call their healthcare provider immediately.
The Procedure of COVID Rapid Antigen Testing
Rapid antigen tests are performed in a clinical setting or in the comfort of your home.
Patients are given instructions by the test manufacturer on preparing themselves for the test on how to cleanse their hands before the procedure, perform the test, read the results, and dispose the kit.
The test itself is quick and simple.
First, a swab is taken from the nose or throat using a sterile, disposable sampling device. Then, the sample is released into a liquid and transferred to a disposable cartridge. On the device, specific antibodies that recognise the structural elements of the virus have previously been attached with a chemical process. The presence of the virus in the collected specimen will be detected and displayed as a coloured band on the device. Results are available within 15 minutes.
Patient Recovery after COVID Rapid Antigen Testing
COVID-19 RATs do not require hospitalization.
Patients are advised to rest and drink plenty of fluids. Once the test is complete, patients will dispose the kit containing the swab, collection tube, and device using the biohazard bag provided.
In the case of a positive result, if the patient is at the clinic, they will be sent directly to the appropriate isolation unit; if the testing site is their home, they will be required to self-isolate immediately and confirm the result with a molecular test.
As of June 2020, the CDC reports that over 3 million COVID-19 tests have been administered in testing sites across the US.
While this number represents a significant increase from previous weeks, it still falls short of the estimated need. More rapid testing kits are expected to become available soon, and as a consequence, the number of molecular and antigen tests performed will likely double again.
While COVID-19 RATs provide a valuable service in screening and diagnosis current and past cases of COVID-19, they are not perfect.
Nevertheless, a screening program may be very helpful to identify an asymptomatic person within the school, workplace, or the community, and act accordingly.
If you suspect that you have been exposed to COVID-19, it is recommended that you consult your doctor for further advice.
Please remember that a negative screening test does not necessarily mean that you have not been infected as the virus may take longer to manifest and the symptom onset is delayed.
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