UPDATE 1-Hologic device effectiveness unclear -- FDA panel

(Adds comments from panel members, context, byline)

By Kim Dixon

GAITHERSBURG, Md. Dec 13 (Reuters) - A U.S. expert advisory panel expressed concerns on Thursday about whether an experimental sterilization device made by Hologic Inc (HOLX.O) is effective, calling the company's data inconclusive.

Although the advisers to the U.S. Food and Drug Administration said the device is generally safe, a majority had doubts about whether the company's stated "success rate" in preventing pregnancies can be generalized to the population, because of problems with the study's design.

The panel is considering the merits of the device system, called Adiana, which uses a catheter to implant a silicone device in the Fallopian tubes to prevent pregnancy.

The FDA is not bound to accept the advice, but typically it does.

"There is some difficulty with a risk-benefit analysis because of a concern about the benefit, and the effectiveness of this technology," said the panel's acting chairwoman, Marcelle Cedars of the University of California.

The panel conceded the company met a pre-specified study goal of a less than 5 percent pregnancy rate.

The panel did not take official votes on the device, which is not its typical practice.

Many experts were troubled that many patients were excluded from the company's effectiveness analysis, which could have skewed the pregnancy rate.

Of 645 women on whom the procedure was attempted, 553 were evaluated for the company's main effectiveness goal. The difference includes those lost to follow up, or excluded for other reasons.

"I am just concerned that the rates we are seeing here won't hold up in actual use," said Ralph D'Agostino, a panel member and statistician at the Department of Math and Statistics at Boston University.

Six patients in the company's clinical trials became pregnant after a year and 10 patients total were pregnant after four years, FDA staff said. The most common adverse events were cramping, vaginal spotting and bleeding, FDA said. (Reporting by Kim Dixon; Editing by Gary Hill/Andre Grenon)