Dialysis provider DaVita Inc. reported fourth-quarter income of $149.4 million, or $1.58 a share, up from $69.02 million, or 70 cents a share, in the same period a year earlier.

Chief executive officer Kent Thiry said during a call that federal prosecutors investigating the company have subpoenaed DaVita executives and board members for testimony. Thiry said they are cooperating and that the subpoenas were expected. 

The average number of daily treatments for DaVita in the United States grew more than 12% to 66,000. The company provided a total of 5.2 million treatments during the quarter.

DaVita operated or provided administrative services at 1,809 outpatient dialysis centers located in the United States serving approximately 142,000 patients and 11 outpatient dialysis centers that are located in three countries outside of the United States. A total of 1,784 centers are consolidated in its financial statements, of which eight centers are located outside of the United States. During the fourth quarter of 2011, DaVita acquired and opened a total of 45 centers, and divested two centers in connection with the acquisition of DSI.