How Deanna's smart watch caught the serious medical condition she never knew she had.

Deanna Recktenwald was sitting in church next to her mum one Sunday when she looked down at the Apple Watch she got for Christmas.

It told the 18-year-old her resting heart rate was between 120 and 130 beats per minute, even though she was sitting perfectly still.

60 to 100 is considered a normal rate.

When her resting heart rate reached 190, the watch told the Florida teenager to seek medical attention. She told her mum Stacey, a registered nurse, who didn’t trust her daughter’s heart rate could be so high.

“She checked my pulse with her fingers and it was spot on every time she checked,” Deanna told ABC Action News.

Stacey added, “It was alarming that the watch was telling us to seek medical attention. I didn’t even know that it had the capability of giving us that alert.”

Watch Deanna’s story below. Post continues after video.

Deanna had been experiencing some shortness of breath prior to her watch’s warning. But she was “a fit, athletic kid,” her mum said.

By the time Deanna was admitted to Tampa General Hospital, staff noticed her heart rate was coming back down. But they took blood and ran a series of tests just in case.

“The blood work came back showing my kidneys were failing,” Deanna told InsideEdition.com.

“It was a complete shock… I was in kidney and liver failure four years ago, but that was due to an infection that spread to my whole body.”

Doctors found Deanna had chronic kidney failure, her kidneys were functioning at only 20 per cent capacity.

“The prognosis isn’t good — she’s going to need a kidney transplant in two to three years,” Stacey said.

After their medical scare at the end of April, Stacey decided to contact her local Apple store. She wanted to write a letter thanking the company for how their smart watch saved her daughter's life.

“I am forever grateful to Apple for developing such an amazing lifesaving product,” she wrote in an email sent to the store manager.

“If it wasn’t for her Apple watch alarming her about her HR we wouldn’t have discovered her kidney issue. I honestly feel that your Apple Watch has saved my daughter’s life.”

Within days, she received a reply from Apple's CEO Tim Cook.

"Stacey, I'm happy to hear your daughter is fine now. Thanks for sharing her story. This inspires us to keep pushing," ABC reports his reply read.

The tech CEO also shared Deanna's story on Twitter, tweeting "stories like Deanna’s inspire us to dream bigger and push harder every day."

Stacey said her daughter is home and doing fine, and Deanna is managing her kidney condition.

She won't be taking her Apple Watch off anytime soon though.

Do you wear a smart watch? Do you pay attention to the data it records?

Do you consider yourself to be healthy? Apparently only seven per cent of the Australian population actually is. Listen below.