To win approval for the $2 billion deal, the companies agreed to sell NxStage's bloodline tubing set business, the agency said. Fresenius and NxStage together dominate the market for the single-use plastic tubes used during dialysis, the agency said.

The five FTC commissioners split along party lines in voting on whether to approve the merger. The three Republicans, Joseph Simons, Noah Phillips and Christine Wilson, voted "yes," while Democrats Rohit Chopra and Rebecca Slaughter dissented.

A spokesman for Fresenius was not available for comment outside of European business hours.

Fresenius and rival DaVita Inc dominate the U.S. market for dialysis clinics. Fresenius, which has 2,200 U.S. dialysis clinics, struck the deal to buy NxStage in August 2017 as a way for it to move into cheaper home dialysis. It has served 190,000 U.S. patients, according to its website.

Simons and the Republican commissioners defended the settlement by arguing that since most patients cannot use in-home dialysis, the two services do not compete against each other, among other reasons.

Chopra and Slaughter, in separate statements, noted that the clinic dialysis market was already concentrated and that the merger could lead to higher prices for people with kidney ailments.

(Reporting by Diane Bartz; Editing by Susan Thomas and Peter Cooney)