Last November, the German company sent a memo to dialysis centers that it operates in the United States, warning them about a link between dosing errors with its GranuFlo product and a higher risk of cardiac arrest.

But FMC did not send the memo to other U.S. dialysis centers that may use GranuFlo, but are not operated by the company.

After learning of the memo in March, the U.S. Food and Drug Administration asked Fresenius to broaden its notification to all customers.

"As in other cases in which FDA learns of product-related hazards, we are evaluating whether Fresenius met its regulatory responsibilities," FDA spokeswoman Morgan Liscinsky said on Friday.

"Should we determine that Fresenius did not do so, Fresenius is subject to compliance action," Liscinsky said.

A spokesman for FMC, based at the group's German headquarters, said the November memo was based on preliminary findings and was meant to prompt further fact-finding within the company.

The FDA's March request was informal and subsequent talks led to FMC informing all clinics, he said. "There is no formal investigation by the FDA and we have no indication that there will be one."

FMC operates about a third of all U.S. dialysis centers for patients with kidney failure, but it also supplies dialysis machines and supplies to rival clinic operators there and in Europe.

The company, which makes almost two thirds of its revenue in North America, is controlled by German healthcare conglomerate Fresenius SE & Co KGaA.

In a November 4 memo to FMC's U.S. doctors, the company said statistics point to doctors giving too much GranuFlo to some patients, triggering cardiac arrest, according to the New York Times, which reported on the issue on Thursday.

"In light of these troubling findings," doctors should dose more carefully, the memo says, and "this issue needs to be addressed urgently."

(Reporting by Ludwig Burger; Additional reporting by Anna Yukhananov in Washington, D.C.; Editing by Erica Billingham and Tim Dobbyn)

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