If you’ve been relying on a Fitbit Surge or Charge HR to track your heart rate at the gym, you might want to take those readings with a grain of salt. According to a recent study [PDF], Fitbit heart rate readings may be both inaccurate and inconsistent. Not only do the biometric devices regularly produce incorrect heart rate readings, the study claims, but two Fitbits used to measure one person’s heart rate are likely to produce different results.

Vocativ explains that researchers at California State Polytechnic University, Pomona tested out Fitbit’s PurePulse heart rate monitoring technology by having 43 healthy adult volunteers undertake different forms of exercise while wearing both a Fitbit (of either the Surge or Charge HR variety) and an electrocardiogram. When researchers compared the readings from the Fitbit and electrocardiogram, they found that Fitbit readings were not only inaccurate, but that inaccuracies increased as participant heart rates rose. That means that the more strenuous your workout, the less reliable your Fitbit readings become. Additionally, researchers found that two Fitbits recording the same heart rate produced readings that varied by as much as eight percent.

The study explains, “Based on our analysis of those readings, we conclude that the Fitbit PurePulse Trackers do not provide a valid measure of the users’ heart rate and cannot be used to provide a meaningful estimate of a user’s heart rate, particularly during moderate to high intensity exercise.”

However, it’s important to note that the study was funded by a group of lawyers who filed a class-action lawsuit against Fitbit. And, while the findings are enough to make anyone wary of their Fitbit heart rate readings, the sample size was small enough that you might want to wait for more research to emerge before you toss your Fitbit in the trash. In general, the study serves as an important reminder that many biometric devices and fitness apps should be confronted with a level of skepticism. Most have not been approved by the Food and Drug Administration (FDA) and there’s little guarantee the readings they provide are accurate. A good rule of thumb is to ask your doctor to recommend a device if there are real medical reasons your readings need to be accurate. Otherwise, enjoy your biometric readings, but keep in mind they might not be the most precise.

[h/t Vocativ]