Berkshire Hathaway, the largest investor in DaVita Health Care Partners Inc, has entered into an agreement to increase its shareholdings to 25% of the US operator of dialysis clinics.

In regulatory documentation filed, the companies have entered into what is known as a 'standstill' agreement to indicate the maximum percentage of shares that Buffett can own without being seen as an unsolicited takeover.

DaVita's share prices increased by US$3.84 or 3.3% to US$121.40 per share in after hours trading when the agreement was announced. Currently, Berkshire owns about 15 million DaVita shares or 14.2% as of March 4 in previous documents filed.

With the valuation, the stake would be well over US$1.76 billion at US$117.56 per share price. This gives DaVita's enterprise value at US$12.4 billion.

In separate news, DaVita reported a first quarter operating profit excluding a legal reserve of US$196.9 million or at US$1.84 per share.