(CNN) Editor's Note: The United States Food and Drug Administration issued a warning letter to online vision test company Opternative in October, 2017, requesting that the company cease activities that result in the misbranding or adulteration of its test as an "eye examination" mobile medical app device. That letter was posted on the FDA website in March, 2018, and a spokesperson for Opternative said in an emailed statement to CNN, "We have responded to the Warning Letter and we are working closely with FDA on this matter."

It happened again: You've misplaced your only pair of glasses. Or maybe you've run through your last box of contacts. In order to replace them, you need a new prescription, and that means, an appointment with your eye doctor.

But now, companies such as Opternative GlassesOn , and Essilor are offering to check your vision online -- without having to step into a doctor's office.

Opternative even sends your vision test results to an ophthalmologist licensed in your state who can review the results and your prior prescription and issue a new prescription for glasses, contacts or both.

Yet, online vision tests are not meant to replace comprehensive eye health exams, and some eye care professionals question whether they are beneficial.

I took an online vision test, received results and then asked Dr. Tom Spetalnick, clinical director of Woolfson Eye Institute in Atlanta, to conduct an in-person vision test.

measure a patient's prescription for eyeglasses or contacts. Vision tests, also known as refraction tests measure a patient's prescription for eyeglasses or contacts.

Spetalnick, a member of the Georgia Optometric Association , took me through a comprehensive eye exam as if I were a candidate for Lasik eye surgery, which he specializes in. During the exam, he was unaware of my online results.

It turned out that the online vision test and the in-person exam produced the same prescription results, revealing that my prior eyeglass and contact prescriptions had not changed.

The online test did not identify signs of astigmatism.

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Dr. Steven Lee, co-founder and chief science officer of Opternative, said there are times when the company's vision test will find a small amount of astigmatism that other professionals have missed, and vice versa.

"The bottom line with subjective refraction is, it's dependent on how the patient is responding," Lee said. "I'm very confident in how our practitioners and how ophthalmologists who work with us prescribe prescriptions in the end."

Other eye care professionals, however, have expressed concern about the accuracy of online vision testing and whether patients might forgo comprehensive eye exams in favor of more convenient -- but less thorough -- online tests.

The difference between vision tests and eye exams

This new wave of online health services, or those received through telephone or videoconferencing, is described as telemedicine

"The way I see telemedicine is, it's a more unique method of delivering health care to individuals, whereby it's a lot more accessible," Lee said. "Unlike the typical brick and mortar type locations where you need to make an appointment and go in, telemedicine allows individuals to seek care at their convenience."

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Spetalnick said, "there is such a thing as telemedicine, and it's a positive," but he added that he has some concerns about where online vision testing fits in.

"Eye care patients tend to think that an eye exam is just a measurement for glasses, and that is a real important test, because people want to see well out of their glasses. But it's a fairly small part of an overall comprehensive eye examination and has no impact, or very little impact, on assessing the health of the eye," Spetalnick said.

"There is a standard of care when an eye doctor is expected to perform an eye exam and the eye exam is expected to determine if the patient does or doesn't have things like glaucoma, retina problems, even diabetes can be detected," he said.

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Opternative states on its website that it's "not a replacement for a comprehensive eye health examination." Rather, it's a "device that can only be used to prescribe glasses and contact lens prescriptions by licensed physicians."

So, the service could be used in addition to visiting a doctor's office for comprehensive eye health examinations.

Yet "even if you're talking about nothing but assessing the proper eyeglass prescription for an individual, a single test isn't the way a doctor ought to be determining what to prescribe for that patient. You need to collect multiple data points," Spetalnick said.

"Frequently, you also have to dilate the patient and do that test all over again to see if that patient's actually focusing in and out appropriately," he said.

Based on the way online vision tests are designed, however, there is no need for dilation, Lee said in response. "The way it's different is, it quantifies how you see," he said. "So, it basically is able to measure your visual perception, how you see out of your eyes, without any correction."

Inspired by a patient to go online

It was a patient who motivated Lee, in 2009, to develop an online vision testing company . At the time, he was working as an optometrist at a private practice in Chicago and teaching ophthalmology students at Northwestern Hospital.

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"She was a single mom. It sounded very difficult for her to make it into the clinic, and her vision was changing and fluctuating a lot," Lee said of the patient, who needed refraction tests.

"She finally makes it in, and then she asked me this one question that really just stuck with me. ... 'Why can't we do eye exams at home?' and that just really gave me a light-bulb moment," said Lee, who has a background in engineering, optics and eye care.

"I ended up going home and just started doodling, that night, a lot of different ideas," Lee said. He then realized that refraction tests could be conducted online, though it would be controversial.

"I realized that it really was possible and then figured out the concept," he said.

Taking an online vision test

Taking the Opternative online vision test requires internet access, a computer, a smartphone and 10 to 25 minutes of your time.

After setting up an account, you are asked to look at your computer screen and read various displays of letters, shapes and numbers, based on traditional eye charts. You indicate what you can see and what you can't by responding via your smartphone.

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Some portions of the test require that you step 10 feet away from your computer screen.

"Using a computer and a smartphone, it's basically a two-screen experience that allows the two to become linked together," Lee said.

Once you finish the test, you pay a fee to have your results sent to an eye doctor who is licensed in the state where you completed the test.

The doctor then reviews the results, looking for signs of nearsightedness, farsightedness, or astigmatism based on the test's algorithms and how the test quantified your visual perception. The doctor also assesses your medical history, pre-existing conditions, and prior prescription, before issuing a new prescription.

Medical or vision insurance typically isn't accepted.

Many eye care professionals still aren't convinced that patients are getting a good value with online vision tests, "because first of all, you're not getting an eye exam, and you're talking yourself out of getting proper eye health care," Spetalnick said.

"For $40, $20, $1, it's a bad call if you're getting an eye test instead of a comprehensive eye examination," he said. "If you want an inexpensive eye exam, they're out there."

Patient protection safeguards to discourage online testing and encourage in-person comprehensive eye exams have been enacted in some states, including Georgia, Indiana, Michigan, Oklahoma, South Carolina and West Virginia, according to the American Optometric Association

Currently, Opternative is available for use in 39 states

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"For us, going forward, we're really open to conversations within the industry. We'd love to keep open dialogues with the optometric community and the state legislative boards, because we really want to provide this great service and technology to the masses in North America and also the rest of the world," Lee said. "We think that there could be a lot of benefit, and we just want to work with everyone."

If you live in a state where online vision tests are provided, the American Academy of Ophthalmology offered some recommendations.

Online vision testing may be appropriate for adults, 18 to 39, without severe corrective eyeglass prescriptions and symptoms of eye disease.

Online testing is not recommended for people under 18, or 40 and older.

People with symptoms of eye disease should see an ophthalmologist, a medical doctor who treats eye disease and conditions.

In a 2015 news release , the academy said it is generally supportive of new technologies because they can help make diagnostic tools more available in remote areas and reduce health care visits and costs.

But, as with any new medical technology, the academy noted that online vision testing needs to be evaluated over time for safety and efficacy.