The following briefly describes the BinaxNOW Ag Card test, one of the most commonly used at-home self-test kits.*
BinaxNOW™ Ag Card Test:
The BinaxNOW™ Ag Card test is an EUA FDA-approved home test manufactured by Abbott and available at several retail outlets. The test uses a lateral flow immunoassay (LFIA) technology to qualitatively detect the nucleocapsid protein antigen from SARS-CoV-2 collected in direct anterior nasal swabs from individuals with or without symptoms. Also, it can be used in individuals for other epidemiological reasons to suspect COVID-19 infection when tested twice over a three-day period with at least 36 hours between tests. The test is authorized for non-prescription home use with self-collection by direct anterior nasal swabs from individuals aged 15 years or older or for individuals aged two years or older who use an adult to collect an anterior nasal swab specimen.
Using an LFIA technique, the test uses an immunochromatographic membrane system that has highly sensitive antibodies to detect SARS-CoV-2 nucleocapsid protein from the nasal swab specimens. The membrane has SARS-CoV-2 specific antibodies and a control antibody immobilized onto the membrane support as two distinct lines and contains other reagents/pads that serve to construct a test strip. The test strip and a well to hold the swab specimen are mounted on opposite sides of a cardboard test card.
The test is performed by adding drops of extraction reagent to the swab well and then inserting the patient specimen into the test card through a hole in the swab well. The swab is then rotated three times clockwise, and the card is closed, bringing the extracted sample into contact with the test strip. The reaction is then allowed to proceed for 15 minutes, after which the test results are interpreted by visually examining the presence or absence of detectable pink/purple-colored lines. The results can be read within a 15-minute time frame but should not be read after 30 minutes.
The test strip has a sample and control line, which is read visually. A positive test is indicated by a pink or purple line on both the sample and control strip lines, whereas a negative test is identified by a sample line without any color, and only the control line has a pink/purple color.
When performing the test, there are certain considerations and limitations to take into account, including the following:
The BinaxNOW test has an accuracy of 84.6% sensitivity and 98.5% specificity, which could suggest the possibility of a false-negative result. The test could yield a false negative result, especially if the specimen collection of the nasal swab is incomplete or the test is run too early in the infection, or an operational error occurs when performing the test.
While a positive test indicates the presence of the SARS-CoV-2 viral antigens, the results should be correlated with clinical information, such as past medical history and other diagnostic information or co-infection with other viruses. In addition, a negative test result should be treated as presumptive, and confirmation with a molecular assay may be necessary for patient management.
The test limitations indicate that negative results do not rule out SARS-CoV-2 infection and should not be used as the sole basis for treatment or patient management decisions, including infection control decisions. If serial testing is performed, then additional confirmatory testing with a molecular test for negative results may be necessary.
