When BinaxNOW shows a positive line, you can almost always believe it.1PubMed Central. Diagnostic Accuracy of the Abbott BinaxNOW COVID‐19 Antigen Card Test, Puerto Rico A negative is a different story. The test needs a fairly large amount of virus in your nose before it turns positive, so it can miss infections that are still small or just getting started.2PubMed Central. Analytical Sensitivity of the Abbott BinaxNOW COVID-19 Ag Card That makes it best at answering whether you are likely contagious right now, not whether you have any trace of infection at all.
What a Negative Result Can and Cannot Tell You
BinaxNOW detects a protein from the coronavirus’s outer shell from a nose swab.3PubMed Central. BinaxNOW COVID-19 Ag Card Product Insert Because it looks for that virus protein directly, it needs more virus than a PCR test, the lab test that copies the virus’s genetic material until it is easy to detect.4PubMed Central. Analytical Sensitivity of the Abbott BinaxNOW COVID-19 Ag Card The lab test can pick up about 100 times less virus than BinaxNOW.5PubMed Central. Analytical Sensitivity of the Abbott BinaxNOW COVID-19 Ag Card That gap is why a card can read negative while a lab test would have found something.
Across studies of adults with symptoms, BinaxNOW caught about 87 out of every 100 infections the lab test found.6PubMed Central. Comparison of diagnostic accuracy of rapid antigen tests for COVID-19 compared to the viral genetic test in adults: a systematic review and meta-analysis Some real-world sites saw much lower rates, as low as about half of lab-confirmed infections.7PubMed Central. A Real-World Comparison of SARS-CoV-2 Rapid Antigen Testing versus PCR Testing in Florida
The studies disagree because each one tested different people at different stages of illness. Sites with many early or mild cases report lower catch rates. Swab technique and who read the card varied too.
A catch rate of about 87 in 100 means that if 100 people with symptoms really had COVID, roughly 13 would still get a negative result. The card alone can’t tell you which group you fall into.
How far it moves the odds depends on how likely COVID was before the test. If you have a sore throat and fever, were around someone who tested positive, and COVID is circulating locally, a single negative deserves less trust than it would if you have a mild allergy-like tickle and no exposure.
A negative card means only that the test did not find enough virus protein in that swab at that moment, so you may still be able to spread the virus. Wearing a mask around others until a repeat test is a reasonable precaution, and feeling worse the next day is a good reason to test again rather than lean on yesterday’s card.
Why a Positive Result Is Almost Always Right
In people without COVID, BinaxNOW correctly showed a negative about 99 times out of 100.8PubMed Central. Multidisciplinary assessment of the Abbott BinaxNOW SARS-CoV-2 point-of-care antigen test in the context of emerging viral variants and self-administration That held whether or not people had symptoms.9PubMed Central. Diagnostic Accuracy of the Abbott BinaxNOW COVID‐19 Antigen Card Test, Puerto Rico In one community study, every positive result in people with symptoms was confirmed by the lab test.10MMWR Morbidity and Mortality Weekly Report. Evaluation of Abbott BinaxNOW Rapid Antigen Test for SARS-CoV-2 Infection at Two Community-Based Testing Sites — Pima County, Arizona, November 3–17, 2020 So if you see a positive line, treat it as COVID and isolate.
Any visible pink line in the sample spot counts as positive, according to the instructions. A faint line usually means less virus, not a doubtful result. Read the card within the time window listed in your instructions, since the card can change after that.
Right after a positive, tell people you were recently close to and stay away from others as current public health guidance advises. If you are older, pregnant, or have conditions that raise your risk, call a clinician soon, because antiviral drugs work best when started early.
A lone positive with no symptoms and no exposure deserves a bit more care. When almost nobody around you has COVID, even a rare false positive makes up a larger share of all positives, so a surprise result is somewhat less certain. A positive in someone with symptoms or a known exposure is much more convincing, because COVID was already likely. In the unexpected case, repeating the test with a fresh kit or getting a lab test is reasonable, and meanwhile it makes sense to stay away from others. With symptoms or a known exposure, you don’t need a lab test to act on a positive. A doctor can help if a positive would change your treatment or work plans.
Why Having Symptoms Makes the Test More Accurate
At one testing site the test caught 87 of 100 infections in people with symptoms but 71 of 100 in people without.11PubMed Central. Implementation and Accuracy of BinaxNOW Rapid Antigen COVID-19 Test in Asymptomatic and Symptomatic Populations in a High-Volume Self-Referred Testing Site In another community study it caught only about 36 of 100 infections in people without symptoms.12MMWR Morbidity and Mortality Weekly Report. Evaluation of Abbott BinaxNOW Rapid Antigen Test for SARS-CoV-2 Infection at Two Community-Based Testing Sites — Pima County, Arizona, November 3–17, 2020 People tend to carry more virus in their nose when they are sick, which is why the numbers drop when symptoms are absent. The figures vary because “no symptoms” covers very different people: those just before symptoms begin, those who will never have them, and those near the end of an infection.
Exposure changes the picture. Among symptom-free people who had a known exposure, it caught about 82 of 100.13PubMed Central. Implementation and Accuracy of BinaxNOW Rapid Antigen COVID-19 Test in Asymptomatic and Symptomatic Populations in a High-Volume Self-Referred Testing Site Someone exposed but well might be early in infection, with virus still building, or might never be infected at all. A negative can mean either, which is why one test is not enough. A negative after exposure but before symptoms should be repeated.
Children are a bit different. In one study of over 1,000 children, it caught about 93 of 100 infections, and results were similar with and without symptoms. A negative for a child is somewhat reassuring, but it is still not certain and should be repeated after exposure or with symptoms.
Whether BinaxNOW Tells You If You’re Contagious
When people carried large amounts of virus, BinaxNOW caught about 99 of 100 infections.14PubMed Central. Performance and Implementation Evaluation of the Abbott BinaxNOW Rapid Antigen Test in a High-Throughput Drive-Through Community Testing Site in Massachusetts At lower virus amounts, it caught only about 81 of 100.15PubMed Central. Performance and Implementation Evaluation of the Abbott BinaxNOW Rapid Antigen Test in a High-Throughput Drive-Through Community Testing Site in Massachusetts Among samples where live, spreadable virus could be grown in a lab, BinaxNOW was positive in about 88 of 100.16Emerging Infectious Diseases journal. Effectiveness of Abbott BinaxNOW Rapid Antigen Test for Detection of SARS-CoV-2 Infections in Outbreak among Horse Racetrack Workers, California, USA A positive BinaxNOW was also more likely to contain growable virus than a positive lab test (57 vs 35 of 100).17PubMed Central. Performance Characteristics of the Abbott BinaxNOW SARS-CoV-2 Antigen Test in Comparison to Real-Time Reverse Transcriptase PCR and Viral Culture in Community Testing Sites during November 2020 Lab tests often pick up leftover traces of virus from people who are no longer very contagious. The lab test can detect virus fragments after you have recovered, while BinaxNOW often turns negative sooner as virus levels drop. Most spread happens when virus levels are high, which is the range BinaxNOW detects best, so a positive antigen test is a reasonable stand-in for “I could be passing this on.”
A negative makes it less likely you are highly contagious right now, but it doesn’t remove the chance. Virus levels rise over the first days, so that can change within a day. If you have symptoms, it is safest to wear a mask and keep your distance until they improve or a repeat test is also negative.
How Timing and Repeat Tests Change Your Result
Early in an infection, there may be too little virus in your nose for the test to find. In one Omicron-era report, the test caught none of the infections in the first 48 hours after a lab test turned positive.18PubMed Central. Rapid antigen testing for COVID-19: Decreasing diagnostic reliability, potential detrimental effects and a lack of evidence to support continued public funding of community-based testing Among people with symptoms, though, catch rates were similar in the first two days of symptoms (81 of 100) and days 3 to 10 (78 of 100).19PubMed Central. Field assessment of BinaxNOW antigen tests as COVID-19 treatment entry point at a community testing site in San Francisco during evolving omicron surges
Taking a second test the same day barely improved accuracy, from about 77 to 81 of 100.20PubMed Central. Performance of Repeat BinaxNOW Severe Acute Respiratory Syndrome Coronavirus 2 Antigen Testing in a Community Setting, Wisconsin, November 2020-December 2020 A second swab that day faces the same detection limit. After a day or two, the virus may have multiplied past that limit, and the repeat can turn positive. If your first test is negative and you have symptoms or an exposure, wait 48 hours and test again.
After an exposure, avoid testing immediately, since virus levels are still low. Test once a few days after contact, repeat 48 hours later, and test again if symptoms appear.
Each BinaxNOW box includes two tests for exactly this reason. If you have just one left, save it for a repeat rather than using it several times in a day.
A lab test makes more sense in a few situations: symptoms are strong but repeated home tests are negative, you are at high risk and may need antiviral treatment, or an employer or travel rule requires a lab result.
How to Swab and Read the Card Correctly
Using fewer than 6 drops of the liquid can cause a false negative.21FDA. BinaxNOW COVID-19 Ag Card Home Test Instructions for Use So can not twirling the swab inside the card, or putting the swab back in its paper wrapper before testing.22FDA. BinaxNOW COVID-19 Ag Card Home Test Instructions for Use The test parts should also be at room temperature before use. Read the instructions in your box first, because details can differ between kit versions.
Swab the front of the nose, not the cheek or throat.23medRxiv. Direct Comparison of SARS-CoV-2 Nasal RT-PCR and Rapid Antigen Test (BinaxNOW™) at a Community Testing Site During an Omicron Surge Cheek swabs missed 91% of infections that nose swabs found.24PubMed Central. Comparison of SARS-CoV-2 Reverse Transcriptase Polymerase Chain Reaction and BinaxNOW Rapid Antigen Tests at a Community Site During an Omicron Surge: A Cross-Sectional Study Throat swabs caught fewer than half of infections that nose swabs caught.25PubMed Central. Comparison of SARS-CoV-2 Reverse Transcriptase Polymerase Chain Reaction and BinaxNOW Rapid Antigen Tests at a Community Site During an Omicron Surge: A Cross-Sectional Study For good technique, insert the swab about half an inch into each nostril and rub firmly around the inside several times. Count the drops out loud, twirl the swab in the card as instructed, and close it promptly.
Reading matters too. In one study, about 5 in 100 people missed a positive that trained staff could see on their own card.26PubMed Central. Self-administered versus clinician-performed BinaxNOW COVID rapid test: a comparison of accuracy Check the card under good light, and have someone else look if you are unsure.
Does BinaxNOW Still Work on Newer Variants and Older Kits
In lab testing, BinaxNOW needed about the same amount of virus to turn positive across variants.27PubMed Central. Accuracy of 2 Rapid Antigen Tests During 3 Phases of SARS-CoV-2 Variants In one comparison, its accuracy dipped only slightly with Omicron, from about 71 to 70 of 100.28PubMed Central. Performance Evaluation of Five Rapid At-Home COVID-19 Antigen Tests against the Omicron Variant Newer variants are not a reason to distrust the test. One caveat: a variant that produces lower virus levels early could make the test seem to miss more infections without the test working any worse.
That likely explains why people sometimes think the test stopped working. With Omicron, many people tested negative early in illness and positive a day or two later. The pattern reflects timing, so repeat after 48 hours if you have symptoms or an exposure.
Storage matters more than the printed date alone. Store kits between about 36°F and 86°F. Humidity, heat, and storage conditions can affect how well the test performs.29PubMed Central. Accuracy of Expired BinaxNOW Rapid Antigen Tests Don’t leave kits in a hot car or a freezing garage, which may cause invalid or falsely negative results. If a kit has been frozen or overheated, replace it.
The FDA extended the shelf life for many BinaxNOW lots after stability testing. Check the FDA website for the updated date for your lot number, and use the newest kit you have when results really matter.