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หน้าหลัก | สุขภาพดี | สุภาพสตรี | การแปลผลเลือด | โรคต่างๆ | วัคซีน
If you have been in contact with someone who has tested positive for COVID-19 and you think that you may also be infected, it’s recommended that you test every day for a few days.
The virus builds up in your body over a certain amount of time so it may be that the test does not detect the virus in the very early stage of the infection.
Testing frequently is recommended.
If you have tested positive, it is strongly recommended to follow guidelines and procedures set out by your local authorities.
Get in touch with your usual doctor, GP or healthcare professional, they will advise you on the next steps to take
The result of an antigen rapid test is excellent although you can never be 100% sure. Even if your result is negative, it is still advised to maintain social distancing guidelines and any other guidelines and procedures set out by your local authorities.
Introduction and Background
Two testing modalities, molecular and antigen tests, are currently used for the diagnosis of SARS-CoV-2 infection. Molecular testing (e.g., PCR) has been used since the pandemic began and is considered the current “gold standard” by the Centers for Disease Control and Prevention (CDC). The Virginia Department of Health (VDH) recommends that the most sensitive and specific test be used for COVID-19 testing where possible. However, COVID-19 antigen tests, many of which are also point-of-care assays, have a role in diagnostic testing.
This document provides VDH’s current recommendations regarding the use, evaluation, and interpretation of COVID-19 antigen tests. As of May 27, 2021, twenty-five antigen tests have received Emergency Use Authorization (EUA) by the U.S. Food and Drug Administration (FDA). Antigen tests fall into several categories—(1) point-of-care prescription tests designed for use in a medical or testing setting (CLIA waiver is required), (2) point-of-care prescription tests designed for at-home use (CLIA waiver not required), (3) point-of-care non-prescription serial antigen at-home tests (CLIA waiver not required), and (4) prescription antigen tests designed to be run in a moderate or high-complexity lab (CLIA certificate required). For more information about available antigen tests, see FDA’s website about antigen tests for SARS-CoV-2.
These interim recommendations for the clinical use and interpretation of antigen tests are not mandates or requirements. Medical providers should use their clinical judgment, as appropriate, to make the best clinical decision(s) for their patient(s). The clinical guidance provided in this document reflects subject matter input and consensus from VDH. As additional information and data about antigen testing become available, it’s possible these recommendations will change.
Use of Antigen Tests
COVID-19 antigen tests are designed for the rapid diagnosis of active infection primarily by detecting the nucleocapsid protein antigen of the SARS-CoV-2 virus (the virus that causes COVID-19) via nasal swabs or similar clinical specimens. After specimen collection, point-of-care antigen tests take approximately 15 minutes to finalize a result.
Antigen tests are generally less sensitive than molecular tests, and their clinical performance depends on the circumstances in which they are used. Because of the lower sensitivity, false negative antigen test results are possible and testing is most accurate when there is a high pre-test probability of SARS-CoV-2 infection (e.g., high prevalence of infection in the community, a patient whose clinical picture is consistent with COVID-19, etc.). In addition, viral carriage is highest early in infection; therefore, antigen tests are more likely to detect a true positive early in infection.
Many antigen tests are meant to be used within 5–7 days after symptom onset, though specific tests such as the LumiraDx SARS-CoV-2 Ag Test allow for testing up to 12 days after symptom onset. It is important for the healthcare provider to refer to the manufacturer’s instructions for the appropriate time window for the test being used. Test manufacturer’s Instructions for Use (IFU) can be found on FDA’s website on antigen diagnostic tests for SARS-CoV-2.
CDC has released general guidance for antigen testing for SARS-CoV-2. To address one of the highest risk locations for transmission of COVID-19, the CDC also developed considerations for use and interpretation of SARS-CoV-2 antigen testing results in nursing homes, which includes testing information depending on the clinical presentation and the epidemiologic context.
Previously, the primary use of antigen testing was for diagnostic purposes in people suspected of having COVID-19 based on symptoms/signs of the illness. However, in late March / early April 2021, FDA issued multiple EUAs for serial antigen testing products. Although the same testing devices are used, serial antigen tests have important differences. First, serial antigen tests consist of doing two antigen tests over a 2–3 day time period, with an interval of 24–36 hours between tests. Each test manufacturer has its specific instructions. Second, FDA expanded the indications for these tests—they can be used for diagnostic purposes in people suspected of having COVID-19 based on symptoms or signs of the illness, or for screening testing in asymptomatic people or those who have no specific epidemiologic reason (e.g., a close contact) to suspect COVID-19. It is known that about 40%, or more, of people with COVID-19 have no symptoms/signs. Screening testing can identify people with asymptomatic disease so that individual and public health action can be taken promptly. Third, a number of serial antigen test products have been designated as nonprescription items. FDA’s authorization of serial antigen testing products greatly expands the indications for testing as well as public access to testing products. It should be noted that the clinical performance of using serial antigen testing as a means of screening for COVID-19 has not been determined. This is noted in the IFU of each serial antigen testing product, and it is stated that a study to support use will be completed.
Providers conducting antigen testing on either symptomatic or asymptomatic populations must be aware of the potential for false negative or false positive results. When the result of an antigen test does not fit the clinical circumstances of the patient, a molecular test is recommended for confirmation of the antigen test result. This recommendation remains in place when using serial antigen testing. Please see the table below for full guidance on the interpretation and follow-up of antigen tests results.
Examples of populations or circumstances where antigen testing could be considered
Evaluating Antigen Test Results
Results should be interpreted with consideration of the pre-test probability of infection, including the patient’s recent exposures and presence/absence of clinical signs and symptoms consistent with COVID-19. The table below summarizes the interpretation of antigen test results. Nursing homes should follow Considerations for Use of SARS-CoV-2 Antigen Testing in Nursing Homes for specific information on infection prevention and control measures, including cohorting.
Table. Antigen test result interpretation for symptomatic and asymptomatic persons
| Test Result | Person being Tested | ||
| Symptomatic Person
(test as close to symptom onset as possible and as recommended by manufacturer) |
Asymptomatic Person with Close Contact# to a known COVID-19 case | Asymptomatic Person without Close Contact# to a known COVID-19 case | |
| Positive | • Current infection
• Prompt isolation until no longer contagious by symptom-based strategy |
• Current infection
• Prompt isolation until no longer contagious by time-based strategy |
• Presumptive current infection
• Prompt isolation while awaiting confirmatory test result • Confirm positive result with a PCR test done in a high-complexity CLIA-certified laboratory+* • Patients with positive confirmatory test should isolate until no longer contagious by time-based strategy |
| Negative | • No antigens were detected
• Confirm negative antigen result with a PCR test done in a high-complexity CLIA-certified laboratory+ • Prompt isolation while awaiting confirmatory test result |
• No antigens were detected
• Close contacts who test negative must still complete 14 days of quarantine^ • Obtain COVID-19 PCR test if person develops symptoms |
• No antigens were detected
• No additional case follow-up necessary • Reinforce prevention measures |
# Close contact is defined as being within six feet of someone known to have COVID-19 for a total of 15 minutes or longer over a 24-hour period, or having exposure to respiratory secretions from an infected person (e.g., being coughed or sneezed on, sharing a drinking glass or utensils, kissing), starting from two days before the person became sick (or two days before specimen collection if asymptomatic) until the person was isolated.
^ VDH and CDC continue to recommend a 14-day quarantine period as the safest option. If the person cannot stay home for the full 14 days after exposure and does not have symptoms, the person may end quarantine earlier. Counting the date of last exposure as Day 0, the person may leave home after Day 10 without testing, or after Day 7 with a negative PCR or antigen test performed on or after Day 5. Close contacts who do not have symptoms of COVID-19 and who have either recovered from COVID-19 or been fully vaccinated for COVID-19 might not need to get tested or quarantine; see here for more information. If the person does not stay home for the recommended 14 days or is not required to quarantine, the person should monitor for symptoms for the full 14-day period after the last exposure and continue to follow all other applicable recommendations (e.g., wear a mask, practice physical distancing, avoid crowds and poorly ventilated areas, and wash your hands often).
*A positive antigen test result in an asymptomatic, unexposed individual should be immediately followed by a PCR test in a high-complexity CLIA-certified laboratory to verify the positive result. This follow-up specimen should be collected within 24 hours of the original test, if possible, and no more than 48 hours after the antigen test. Specimens collected more than 48 hours after the initial test may lead to discordant results. If the confirmatory PCR is negative on an appropriate specimen collected in the proper time frame, and the individual has remained asymptomatic, the antigen test would be considered a false positive and the individual not counted as a COVID-19 case.
+While multiple specimen types may be acceptable, if possible, confirmatory tests should be performed using specimens with evidence of the most sensitivity, such as nasopharyngeal or mid-turbinate swabs.
Clinical management of patients with a positive antigen test should be the same as for patients with a positive PCR test, beginning with prompt isolation. Further public health actions including case investigation and contact tracing should be initiated in coordination with the local health department.
Currently Available POC Antigen Tests
As of May 27, 2021, the following twenty POC antigen tests have an EUA from FDA. More information about each of these tests can be found on FDA’s website for COVID-19 antigen tests.
Reporting Antigen Test Results
All positive and negative antigen results from diagnostic or screening testing must be reported to VDH within 24 hours. The requirements and methods of reporting antigen test results are the same as for molecular tests (e.g., PCR). Laboratories and testing sites are required to report electronically.
VDH has developed a reporting portal for point-of-care (POC) COVID-19 test results. This portal can assist testing sites that lack Electronic Laboratory Reporting (ELR) capability in meeting the requirement of the CARES Act to report every diagnostic and screening test (e.g., molecular, antigen, antibody) performed to detect SARS-CoV-2 or to diagnose a possible case of COVID-19. This portal allows for the rapid entry of person-level test results for positive and negative results, and provides the ability to enter aggregate negative results as necessary for high-volume testing sites.
Sites conducting POC testing will need to register first to utilize the reporting portal. During this one-time registration, facilities will provide site information and select the type(s) of testing equipment utilized. When reporting results, sites will need to provide individual patient information, including demographic information, and the test result. Additionally, specific questions requested by the U.S. Department of Health and Human Services (HHS) can be answered if the information is known.
If a false positive or false negative antigen test result is found, VDH recommends that the findings be reported to FDA through the MedWatch system, and the test manufacturer.
References