OMAHA, Neb. -- Warren Buffett's company bought more than 500,000 more shares of dialysis provider DaVita Inc.'s ( DVA) stock last week after prices dropped because of fears of reduced Medicare payments. The latest purchases reported to the Securities and Exchange Commission on Friday give Buffett's Berkshire Hathaway Inc. ( BRK.A) ( BRK.B) 15.6 million shares of DaVita's stock. Berkshire now owns about 15 percent of DaVita, but it has agreed not to buy more than 25 percent of the Denver-based company unless, both companies agree. DaVita runs nearly 2,000 outpatient dialysis clinics. Berkshire has steadily increased its stake in DaVita over the past two years. Berkshire initially disclosed owning 2.7 million DaVita shares at the end of 2011. DaVita's shares had gained nearly 10 percent this year before last week's proposed 9.4 percent rate cut from the Centers for Medicare and Medicaid Services. Berkshire's investments in DaVita likely drove some of the interest in the stock because so many investors try to copy Buffett's moves. Buffett has said that his investments tend to be the larger ones in Berkshire's portfolio. He says investments smaller than $300 million are likely to be the work of one of Berkshire's two other investment managers. The final Medicare rates won't be determined until October, and any reduction in rates could be phased in over several years. But any reduction would likely cut DaVita's revenue because the company provides dialysis services to about 156,000 patients with chronic kidney failure and end-stage renal disease.