Abbott is warning that sensors on some of its blood sugar monitoring systems may need to be replaced to prevent inaccurate readings.

Testing showed that some sensors on the FreeStyle Libre 3 system may incorrectly report high blood sugar levels, the medical device maker said Thursday. An inaccurate high blood sugar reading can prompt patients to take insulin when they don’t need it.

The devices were distributed in the first half of May in the United States. Abbott estimates that less then 1% of U.S. users are affected.

Customers who live outside the country or use other versions of its FreeStyle Libre system are not affected, the company said.

The continuous glucose monitoring system uses a sensor, a reader and an app to help people with diabetes check their blood sugar without having to draw drops of blood from their fingers. The U.S. Food and Drug Administration first approved the Abbott devices in 2017.

Abbott said it will replace the sensors at no charge. The company said people should check its website to confirm whether their sensor is affected. The sensor came from these three lot numbers, the company said: T60001948, T60001966, T60001969.

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