• Experts say that even a faint line recorded on a positive at-home COVID-19 indicates that you are sick and may be contagious.
  • The opaqueness of the results window and a positive line may also indicate how sick you are, especially if a dark line appears in under 15 minutes.
  • Read on to learn more about faint lines on home rapid antigen COVID-19 tests.

While at-home COVID-19 tests felt like a strange novelty earlier in the pandemic, they're an essential part of our routines these days — and using one is pretty straightforward for most people. Nearly all the COVID-19 home tests currently on the market with an emergency authorization from the Food and Drug Administration (FDA) make use of a control panel and a results window. In most cases, if a secondary line appears in the results window after a nasal sample is submitted, it signals that the test is positive and the test's user could potentially spread COVID-19.

But what about the uncertainty of a faint line that may seem almost unreadable? After all, rapid COVID-19 tests aren't foolproof — user error may occur, certainly. But research highlighted by virologists at Massachusetts General Hospital indicates a false positive is exceedingly rare. They're more likely to happen at the end of a COVID-19 sickness than at signs of first symptoms.

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This may not be spelled out in test instructions or marketing materials included with a rapid test, but experts have established that the shade of the line shown in a positive COVID-19 test — as well as how long it takes to appear — is linked to how sick you are, and how contagious you may be to others, which has been widely reported in the wake of recent Omicron-fueled waves of breakthrough infections.

But here's what many people aren't aware of: Any shade or opaqueness indicates that you are indeed sick. Joseph Mann, MSN, FNP-C, a medical science liaison for global medical technology company BD, explains that the clarity of a results window isn't a perfect measure of how truly infectious or sick a person may be. As confusing as they may be, a light positive line is still considered positive; you'll need to follow the same precautions and care instructions as if your result was clear as day.

What exactly does a COVID-19 home test measure?

There are different kinds of rapid home tests that Americans can use. Certain tests measure for specific proteins (known as nucleocapsid proteins) within a sample that are associated with SARS-CoV-2, whereas others identify genetic material instead. The liquids used within a COVID-19 test help to extract "viral proteins" from a nasal sample, Mann explains, which are then transferred onto the actual home test receptor.

In that 15-minute window, a chemical reaction takes place — and a result indicates whether these proteins were present in the sample. But those taking multiple rapid COVID-19 tests may not realize that their result windows and the opaqueness of a positive line can often vary depending on a myriad of factors.

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"Faint or dark test lines develop on COVID-19 tests for any number of reasons and shouldn't be used as a predictor of disease severity or virulence," Mann tells Good Housekeeping. "Slight differences in sample collection, adherence to testing procedures, and other factors can directly impact the strength of the lines."

Meaning just because you may experience a "light" positive on one test doesn't mean it will remain that way on subsequent or repeat testing, Mann adds.

If I see a faint line on my test, is it positive?

The short answer? Yes. As rapid antigen COVID-19 tests measure the presence of viral proteins, even a faint line positive result indicates that SARS-CoV-2 has had an impact on your immune system. A false positive isn't as likely as a false negative result on a home test early in a person's infection, explains Sandra H. Bonat, M.D., a pediatric expert and virologist with VIP StarNetwork, a mobile healthcare provider specializing in COVID-19 support services.

"A faint line on a home COVID test indicates the presence of COVID-19 virus proteins and should be considered a true positive," Dr. Bonat says. "Since it takes a certain level of virus to cause a positive test, a person should assume they are contagious if they have a positive home test result, even if the line is faint."

If you're not currently experiencing any obvious COVID-19 symptoms (like brain fog!), think about your activity over the last few days — have you been traveling? Visiting crowded, indoor spaces? And try to consider the COVID-19 spread around you; officials at the Centers for Disease Control and Prevention (CDC) have released a county-specific spread indicator to help determine risk. More likely than not, a flurry of viral cases in your area should give you more than enough context to realize you are indeed sick.

A faint line on a rapid antigen test should prompt you to immediately isolate yourself from anyone in your home, and follow-up with your primary healthcare provider as soon as possible for further instructions.

Does a faint line mean you have recovered from COVID-19?

If you're testing yourself after the bulk of your COVID-19 symptoms has settled, you may be surprised to see a faint line and a positive result given your recent recovery. But you should consider yourself still contagious if you're testing positive (however faint an indicator may be!) after recovering from COVID-19 symptoms, even if you are outside the five-day minimum isolation window as recommended by the CDC.

"Just as some people will test positive on a home test and remain asymptomatic, some individuals will test positive on a home test after COVID-19 symptoms have subsided," Dr. Bonat adds. "A faint positive line on a home test after recovering from COVID-19 symptoms is an indication of viral proteins being present — therefore, that person may be still contagious."

While experts expect that individuals are less contagious after recovering from primary COVID-19 symptoms compared to the beginning of an infection, Dr. Bonat says it's best to continue to take precautions like wearing well-fitting masks in public and avoiding those who are at elevated risk, in addition to staying home if at all possible, until you've tested negative.