People who wear contact lenses face the same ritual, annually or however long they can stretch it out: They have to visit their optometrist or ophthalmologist to have their vision checked and their eyes examined before they can buy more lenses. A few startups are out to change this routine by giving users an eye exam on their computer or smartphone. But is that legal? It depends on where you live.

Eye exams by phone are just another form of telemedicine, after all, which is interacting with a medical professional remotely. Telemedicine services are limited — you couldn’t have surgery or a biopsy over a webcam, for example.

It’s hard to argue with technology that lets people all over the world have access to the best medical care providers, and a number of states have made eye care in app form along with other forms of telemedicine legal in recent years.

Let’s break this down: What do the services do, and where do you have to live to get them?

What’s an eye exam, anyway?

Optometrists and ophthalmologists have different training, but both types of docs can prescribe glasses and contact lenses. Beyond that, to oversimplify, ophthalmologists have medical degrees, and can perform surgeries and provide more services. Retail “eye doctors” are generally optometrists.

If you opt for contact lenses, you’ll be told that your prescription expires within a year (or whenever you run out of disposable lenses) and you’ll need to have your eyes examined to get a new prescription and any more lenses. This is not necessarily true, and how long your prescription is valid varies by state.

Both types of eye doctors are not happy about the prospect of competition from apps. The American Optometric Association tells patients that online eye tests can be inaccurate, and that the vision test is not the only important part of an eye exam.

“It is not unlike taking a blood pressure reading at a kiosk and expecting a prescription; the reading does not provide sufficient information to determine a patient’s needed course of therapy,” the professional group grumbles.

A full eye exam consists of a discussion of your general health, a refraction test that checks your actual vision, and a visual inspection of your eye and how healthy it is. These can all be performed remotely, but parts of the exam that can’t include an eye pressure test to screen for glaucoma, and an examination that includes dilating the pupils.

At the end of your exam, your doctor should give you a copy of your prescription without being asked so you can fill the prescription at the provider of your choice.

This has been the part of a long-running feud between doctors and contact lens discounters. The two sides have fought over giving patients their prescriptions and over industry-wide price-fixing, since doctors’ offices also make money from selling contact lenses right in the office.

Where can you get an online eye exam?

Opternative, the best-known startup, just offers exams. The test itself is free, but you can pay $40 to have a doctor in your state make it legal.

Others, like Simple Contacts, 1800Contacts, and Warby Parker, offer exams but also sell you eyewear to go with it. They cost $10 to $20, or about one-tenth what an in-person exam will likely cost a contact lens wearer.

To fulfill legal requirements, all of the companies have doctors double check prescriptions. If the remote service you want to use doesn’t have one on staff who is licensed in your state, they won’t be able to help you.

Connecticut just passed a law limiting the use of online exams: Patients can’t get their very first prescription that way, and have to have their first renewal in person as well.

Virginia, meanwhile, passed a proactive law this year that will allow optometrists and opthalmologists to prescribe eyeglasses and contacts using video conference or recorded videos as long as the doctor and patient establish a relationship and the doctor takes a medical history.

Yet a 2016 law passed in Georgia specifically bans remote eye exams, and other states may take action to limit their use, as Connecticut did, or ban them like Georgia.

Should you get an online eye exam?

The answer to that question is up to you, your wallet, and the advice of your own eye care professional. The American Academy of Ophthalmology very cautiously said in 2015 that people who are young, otherwise healthy, and who have had a comprehensive eye exam in the last few years might be appropriate users for app-based services.

While online tests for glasses prescription may be okay for adults 18-39 years of age with healthy eyes,” the group’s medical director of Government Affairs said at the time, “they are not appropriate for those with an eye condition or risk factors for eye disease – such as having diabetes or a family history of glaucoma – or for those who have not had a recent eye examination.”

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