Shares of Biosite Inc. (Nasdaq: BSTE) skyrocketed more than 50 percent on Monday after biomedical lab instrument maker Beckman Coulter Inc. (NYSE: BEC) said it would acquire the firm for $1.55 billion.

Shares of the heart disease diagnostic test maker rose 51.1 percent, or $28.31, to $83.69 in afternoon trading on the Nasdaq Stock Market. The acquisition news didn’t help Beckman Coulter shares, which lost nearly 7.2 percent, or $4.81, dipping to $62.28 on the New York Stock Exchange.

Biosite shareholders will receive $85 per share, a 53 percent premium over the company’s Friday closing price.

The companies said they believe that the acquisition will give Beckman a leading position in tests known as immunoassays, which use certain chemicals to check for specific amounts of a substance in the blood or fluids.

Beckman Coulter’s credit rating could take a hit from Fitch Ratings following the announcement. The ratings firm says additional funding information from Beckman is needed. Fitch says a “multiple-notch downgrade” is probable.

“The rating action reflects the material increase in leverage that will be required to finance the transaction,” Fitch said in a statement.

Beckman said in a statement that the acquisition would add an unspecified amount to its earnings next year and maintained its profit outlook for the year excluding the purchase.