We evaluated the Strep B optical immunoassay (OIA; ThermoBiostar, Inc.) for detecting light and heavy group B streptococcus colonization in 1,306 pregnant women. The women were examined at 20 to 32 weeks gestation and were from six countries. Compared to culture, the sensitivity and specificity of OIA were 13.3 and 98.4%, respectively, for light colonization and 41.5 and 97.7%, respectively, for heavy colonization.Heavy colonization with group B streptococcus (GBS) may place women at high risk of delivering preterm, low-birthweight infants (10,11,15). Earlier recognition of such high-risk women would be useful for intervention strategies. Detection of heavy colonization has traditionally relied on culture methods, which may not be feasible in some health care settings and require 1 to 3 days before results are available. Unfortunately, most rapid GBS tests have poor sensitivity (7, 13). The Strep B optical immunoassay (OIA; ThermoBiostar, Inc., Louisville, Colo.) is a rapid method which may have higher sensitivity for dense GBS colonization (14). We evaluated the performance of this test in an international, multicenter study.We conducted the study at institutions participating in the Global Network for Perinatal and Reproductive Health (http: //www.tju.edu/obgyn/rih/global.cfm). Centers were located in Khon Kaen and Bangkok, Thailand; Manila, The Philippines; Harare, Zimbabwe; Yangon, Myanmar; Dublin, Ireland; and Philadelphia, Pennsylvania. Participants were enrolled in the study between June 1999 and May 2001. Women attending prenatal care clinics and at 20 to 32 weeks gestation were eligible. Exclusion criteria included vaginitis symptoms, fever, vaginal bleeding, multiple pregnancy, use of antibiotics in the past 2 weeks, and active labor. Informed consent was obtained. The study was approved by institutional review boards at all study centers and supporting institutions.At enrollment, study personnel conducted an interview and collected specimens. A sample for GBS rapid testing was collected from the cervix and lower vagina using a ThermoBiostar swab. OIAs were read by study site laboratory personnel and by the manufacturer's laboratory personnel. OIA kits and swabs were supplied by ThermoBiostar.Methods for GBS culture and quantitation replicated those used in the Vaginal Infections and Prematurity Study (16). After collection of the OIA specimen, a Dacron swab (Copan swab) was used to collect a specimen from the cervix and lower vagina. Samples were streaked onto blood agar plates using the four-quadrant method and then placed into Lim broth (ToddHewitt broth containing selective antimicrobial agents). A clean catch urine specimen was collected for GBS culture on blood agar. Suspect colonies showing -hemolysis were identified as GBS using latex agglutination. Heavy colonization was defined as isolation of GBS from either urine or direct plating on blood agar. Light colonization was defined as isolation of GBS from Lim broth alone. Dense GBS colonization was defined as growth in the third or fourth quadr...