Put that nasty keto breath to good use, why don’t you.

A new “breathalyzer” for those on the celeb-loved high-fat, low-carb diet will indicate just how well followers are doing on the restrictive plan.

Keyto, founded by (big surprise) two Silicon Valley doctors, this week unveiled its $99 test, which measures acetone on the breath, similar to how alcohol Breathalyzers measure ethanol, Business Insider reports.

The acetone-smelling breath is one of the diet’s biggest pitfalls. Even fervent followers admit it can be unpleasant, sometimes metallic. They say it’s the result of fatty acids metabolizing, or “burning” in the body. But the smell can also be a good indicator of the body breaking down fat for energy, in which case someone is eating the right balance of nutrients for the diet to work, Keyto says.

The plan has its followers eating mostly protein and fat, while limiting carbohydrates — including the kinds of fruits and vegetables dietitians say are part of a healthy balanced diet. It’s especially favored by biohackers and those looking to lose weight fast, but doctors warn that the majority of people will have a hard time sticking with it.

Keyto’s makers, however, say the device can help its followers stay disciplined.

The battery-powered, handheld stick pairs with an app, which can track the dieter’s progress, and (for an extra fee of $96) suggest keto-friendly meals. The breath test has users taking a 10-second exhalation into the device, which will yield a “Keyto Level” ranging from 1 to 8, with 8 meaning “deep ketosis” and 1 meaning not even close. It’s based on how much acetone vapor is detected in the user’s breath.

The test is one of many products, new and old, that the trendy diet’s followers can buy to help them stay on track by using their breath, urine or blood.