Free home testing kits for sexually transmitted diseases are available to women in four Bay Area counties through Thursday.¿

Females age 18 to 30 in Alameda, Contra Costa, San Mateo and San Francisco counties can go to www.iknowsfbay.org to order a test kit shipped to their homes.

They can then mail samples to a laboratory for testing and go online to find out if they have Chlamydia, gonorrhea or Trichomonas.

The confidential tests will be given to up to 400 women as part of a study to determine the effectiveness of a Web-based testing system for sexually transmitted diseases.

National recommendations are that all sexually active women age 25 or younger have an annual test for such diseases, which can cause serious problems if left untreated.

"Many people don't realize they're infected because symptoms are frequently nonexistent," said Dr. Freya Spielberg, senior health scientist for RTI International and director of research mentorship at the Stanford Clinical Excellence Research Center.