Medical device maker Hologic Inc. said Monday that it will buy diagnostic test maker Gen-Probe for about $3.72 billion in an all-cash deal.

Bedford, Mass.-based Hologic will pay $82.75 for each Gen-Probe share.

The price represents a 20 percent premium over San Diego-based Gen-Probe's closing stock price of $68.71 on Friday. Based on the company's about 45 million outstanding shares, the deal is worth about $3.72 billion.

Gen-Probe shares climbed $12.57, or 18.3 percent, to $81.28 in premarket trading. Hologic shares also rose, rising $1.02, or 4.8 percent, to $22.25.

The deal has been approved by the boards of both companies. It is expected to close in the second half of this year.

Hologic said it expects the addition of Gen-Probe to boost its adjusted earnings by about 20 cents in the first fiscal year after the deal closes and significantly more after that.

Gen-Probe provides molecular diagnostics products and services, including several tests for sexually transmitted diseases. Hologic said the company will be a "highly complementary addition" to its diagnostics portfolio.