Shares of HCA Holdings Inc., the largest for-profit U.S. hospital chain, sank in extended trading Monday after the company released preliminary first-quarter earnings data that showed slower admissions growth than a year earlier.

Revenue for the first quarter is expected to be $8.44 billion, almost unchanged from the period a year earlier, the Nashville, Tennessee-based company said in a statement. Analysts had anticipated sales of $8.65, according to the average of 16 estimates compiled by Bloomberg. Results were affected by a slowdown in the rate of growth in admissions and a weakness in outpatient volumes, the hospital operator said.

Volume weakness also was reported April 10 by Health Management Associates Inc., which reduced the high end of its 2013 earnings forecast. HCA, which oversees 162 hospitals and 112 freestanding outpatient surgery centers, Monday reaffirmed its annual earnings forecast of $3 a share to $3.30 a share excluding certain items.

The hospital company said same facility admissions increased 0.1 percent for the quarter, compared with 3.2 percent a year ago, while same facility equivalent admissions declined 0.7 percent compared with a 4.8 percent increase a year earlier.

HCA shares declined 4.7 percent to $35 at 5:24 p.m. New York time after closing down 4.2 percent at $36.72.