The Apple Watch is about to get a lot more useful with bands you can swap out to track health symptoms

• The Apple Watch will link wirelessly to a glucose sensor from Dexcom, a medical device maker, Apple announced at WWDC.
• Dexcom stock spiked more than 6% on the news.
• Developers expect swappable watch bands laden with sensors to track health symptoms.

Apple will release a new bluetooth API for Apple Watch, its heath and fitness-tracking smartwatch. The company's vice-president of technology Kevin Lynch said at the company's annual developer conference that users would be able to link their watch to a glucose sensor from Dexcom, a medical device maker.

Dexcom's stock spiked more than 6% in the wake of the news.

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Health developers say this is a promising sign that Apple will open up new pathways for them to create their own interchangeable watch bands laden with health sensors. Brandon Ballinger, founder of Cardiogram, a health-tracking app for Apple Watch, said it's exactly what developers need to do just that. "This new bluetooth API may seem like a small detail, but with it, Apple is laying the right technical foundation," he explained.

Previously, Ballinger said, developers building health sensors would need to communicate with the iPhone over Bluetooth — but not with Apple Watch. "So data may have been lost if the phone was out of range," he said — and that's a big problem for health conditions that would benefit from continuous tracking.

Health startups are already building watch bands that use sophisticated sensors to track health conditions.

A startup called AliveCor announced in 2016 that it is developing a Apple Watch band called Kardia to track electrocardiograms. The goal is to detect abnormal heart rhythms, called Atrial Fibrillations, via the watch. Such irregularities can result in a stroke and other potentially fatal outcomes. AliveCor is still awaiting clearance from the U.S. Food and Drug Administration before bringing its watch band to market.