The promise of Google's glucose-sensing contact lens is non-invasive continuous glucose monitoring: that rather than having to prick themselves with lancets several times a day, the nation's 26 million people with diabetes could take their readings on their phone via passively collected tears.

If that sounds too good to be true to you, you're not alone. A number of voices in the diabetes care space -- even those already working on "next generation" continuous glucose monitoring -- are skeptical of Google, or anyone else, tackling passive, noninvasive, blood glucose monitoring any time soon.

"That Google Glass thing? That's a science project," Dexcom Executive Vice President of Strategy and Corporate Development Steve Pacelli told MobiHealthNews. "Lots of companies have tried and failed noninvasively to sense glucose in tears. You can measure glucose in tears, the concentration is a lot lower, there's going to be huge time lag issues, the consistency of measurement is going to be a challenge. I don't know how they're going to power it, my guess is they'll power it externally, but if you power it externally, it's not really continuous realtime reporting. ... I'm just not sure that it's reality."

Glooko CEO Rick Altinger agreed, although he sees a little more hope for the project in the long term.

"If you look to Google, they announced they're five years away from a commercial product," he said. "And even then, what will the price point be? Others have tried to use tears and not been successful. Then again, if anybody can do it, Google can do it."

WellDoc Chief Strategy and Commercial Officer Chris Bergstrom added "unfortunately, despite occasional headlines, true non-invasive glucose testing is not around the corner. The regulatory, clinical, technical, and biological hurdles are still too significant. Including for contact lenses."

Dexcom's Pacelli also said that, beyond being overambitious, announcements like Google's were harmful, giving false hope to diabetes patients.

"It's a little frustrating for us who are actually trying to help patients when Google comes out with something that's a science project," he said. "And they kind of admit it. They say 'we're not going to do this on our own, we need to partner with industry, partner with experts.' Ok, fine, but now you've got the mainstream media picking it up just because it's Google, saying 'Google's going to eliminate the need to prick your finger, you can just wear a contact lens around.' It's not fair to patients. It's disingenuous."

But if glucose monitoring isn't coming to the eyeball any time soon, where is it headed? That's a question on which the industry has less consensus, though most are looking to continuous glucose monitoring (CGM).

Monitoring blood glucose continuously (or at least every few seconds rather than every few hours) gives diabetes patients a much clearer picture of their blood glucose trends. This is the realm of technologies like Dexcom's G4 Platinum, which just yesterday received FDA clearance for use in kids, or Medtronic's CGM offering. CGM has been around for about a decade, but the technology has advanced considerably in accuracy and convenience in the last few years. These devices stay in for seven to ten days and check blood glucose through a thin sensor inserted underneath the skin, which connects to a reader device. The device can be set to give off an alarm if the user's glucose gets too high or too low.

Continuous monitoring is particularly useful for type 1 patients, who need regular insulin injections, but Pacelli says it's useful in type 2 patients as well. It's much more expensive than a glucometer and right now patients still have to prick their fingers multiple times per day and use a regular glucometer to calibrate the sensor. However, Pacelli says Dexcom's plan is to eventually replace intermittent glucometers completely. He said Dexcom's fifth generation plan is to connect their existing CGM to a smartphone.

"Then we get to our Generation 6 technology, and that really is the next leap forward in sensor technologies," he said. "It's still an R&D project, but we've certainly tested it in humans and will continue to do so. The goal with Gen 6 is to eliminate the needs for some calibrations, so that ultimately, with [generations] 6,7, and 8, we'd be in the position to eliminate truly the need for patients to prick their finger."

That's a bold statement. For contrast, CGM is used currently by mostly type 1 patients, who make up only about 10 percent of the 26 million people in the United States with diabetes. Of those 2.5 million people with type 1 diabetes, there are only about 150,000 CGM users between Dexcom and Medtronic, the two largest companies in the space.

The Dexcom G4 Platinum

WellDoc's Bergstrom said one of the biggest hurdles is regulatory -- the FDA still doesn't consider CGM accurate enough to prescribe insulin dosing by itself. This is also a hurdle for the "artificial pancreas," a concept that would pair a CGM, an insulin pump, and a computer or app that would give the pump instructions based on CGM readings. Between the difference in cost, and the time it takes to scale up from 150,000 to 2 million, it seems like it will be a very long time before CGM replaces traditional glucose monitoring.

"Dexcom's been around for ten-plus years and it's used by hundreds of thousands of people now, and I think it's a great company and great technology, but it is not used by the masses," Glooko's Altinger said. "There are tens of millions of users out there. [Even] as Dexcom gets more and more accurate, it is not a device that people are going to want to wear. It requires you to insert a sensor under your skin that stays there for approximately seven days. And it's very expensive. No one is expecting that market to grow to tens of millions of people. No one sees that."

Pacelli said that 95 percent of Dexcom's users receive some kind of reimbursement for CGM, and maintains that the device is comfortable and most users don't even notice it's there.

A wearable, accurate, connected CGM would certainly be an improvement in the lives of people with diabetes, and most people believe it's on the way eventually. But CGM achieving anything like the scale of blood glucose monitors in the next five years seems like a pipe dream. Maybe not quite so much of a pipe dream, though, as glucose-sensing contact lenses.

Maybe the future of glucose monitoring is something else entirely. Timelines aside, mobile health advocate and Scripps Health cardiologist Dr. Eric Topol agrees with Pacelli that finger sticks will eventually be replaced -- just not necessarily solely by CGM.

"Ultimately, I think both non-invasive intermittent glucose sensors (not requiring finger sticks) and continuous glucose monitoring will be the only technologies some years from now," he told MobiHealthNews in an email. "Finger sticks for glucose, someday, will be an historical footnote."