Doctors say people who use smart devices to track their health and fitness need to be aware of what the technology can and cannot track.

The Food and Drug Administration has issued a warning that any smart ring or smart watch claiming it can measure one's blood sugar levels without piercing the skin is unable to do so.

Those devices have not been approved or authorized by the FDA.

The concern is that patients will rely on these devices and receive inaccurate measurements and errors, which could be life-threatening.

However, doctors say some smart watches and devices can effectively track other important health and fitness information.

"There's some trackers that only check your step count. Then there's others like watches that can tell you how much sleep you're getting and the quality of your sleep as well as your heart rate, which can be important with someone with a potential underlying heart conditions," says Dr. Laxmi Mehta, a cardiologist with The Ohio State University Wexner Medical Center.

Dr. Mehta says because people have their smart devices on them the majority of time, the devices can sometimes resister potential issues that are not caught by tracking and testing set up by doctors.

"I've had some patients that we've put monitors on them for a whole month and we can't find anything because it's just infrequent, too infrequent. And when they have these devices, we're able to find out like, gosh, you know what they're experiencing is real. It's not anxiety, it's not something in their head. There's something real, but we just can't capture it on our monitors because the monitor is not worn too long," says Dr. Mehta

Cardiologists say smart devices have not been shown to help alert people to a potentially dangerous abnormal heart rhythm called atrial fibrillation.

If the condition goes undiagnosed for a prolonged period, it increases one's risk of having a stroke.

Doctors say tracking one's fitness and health numbers can give them a better understanding of their own health and prompt them to visit a doctor if the numbers they are seeing do not add up.