The FDA has slapped a Class I recall, its most serious and highest recall alert, for strips sold by Roche Diagnostics that are used to check warfarin levels.

The diagnostics arm of the Swiss major has already issued a voluntary recall of certain test strip lots used with its CoaguChek test meter devices.

This recall involves more than 1 million packages of the CoaguChek XS PT Test Strips that were sent out nationwide from the start of the year to the end of October.

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They are used to check the levels of the old blood thinner warfarin, which often requires close monitoring and, initially, dose adjudgments; getting this wrong can lead to higher risks of bleeds, or if under-dosed, not controlling the potential for stroke or other disorders.

Now, the FDA has issued a Class I recall, which means use of these devices “may cause serious injuries or death.” The regulator says that certain test strips used with the devices “may provide inaccurate results and should not be relied upon to adjust the drug dosage.”

In this case, Roche Diagnostics has seen data showing that the test strips may provide results that are higher than the actual International Normalised Ratio, or INR, which is used to measure these levels.

“As a result of incorrect INR results, some patients may be prescribed an insufficient warfarin dose or instructed to interrupt warfarin use, which may increase the risk for dangerous blood clots,” the agency says. About 90 medical device reports and two “serious patient injuries” involving strokes were reported to the FDA.

In a statement, the FDA is warning patients and doctors that they “should not rely” on these test meter devices to monitor warfarin levels if they’re using test strips affected by the recall. Instead, they should have blood drawn from a vein and have their levels measured by a lab test, or use an a different meter device.

“These strips are widely used and we are working diligently to warn health care providers and the public about the dangers associated with this recall. Using faulty strips can lead to serious errors in medication dosage that could cause serious harm or death in some patients,” said Jeffrey Shuren, M.D., director of the FDA’s Center for Devices and Radiological Health.

“We are also working with the company on the swift removal of the recalled strips and to ensure the new corrected strips are distributed to patients and health care providers as quickly as possible.”

Roche says the issue comes from a recent “re-calibration of the test strips to a different international standard,” which happened earlier this year.

They plan to provide new batches of re-calibrated test strips, based on the previous international standard, to their customers by the end of November.