A prenatal screening test for group B streptococcus (GBS) is used to detect the presence of this bacteria in the vagina or rectum of pregnant women. About 25% of pregnant women are colonized by GBS. Screening is used to determine the risk that they will pass the bacteria to their newborns during birth, possibly causing a serious infection.

The Centers for Disease Control and Prevention (CDC) released updated recommendations in 2010 for the prevention of perinatal group B streptococcal disease. The guidelines outline procedures for the universal screening of pregnant women at 35 to 37 weeks gestation to detect GBS colonization. The report also provides details on the recommendation for antibiotic treatment during labor for those who are colonized in order to prevent early-onset GBS disease in newborns. The guidelines contain provisions for:

• Screening all pregnant women for vaginal and rectal GBS colonization at 35 to 37 weeks gestation unless GBS has already been isolated from a urine sample during the pregnancy or the woman has had a previous infant with invasive GBS disease; in these circumstances and when GBS is positive, the woman should be treated with antibiotics during delivery.
• Testing of those who have preterm labor and/or premature rupture of amniotic membranes at less than 37 weeks gestation; those whose membranes have ruptured more than 18 hours previously are at an increased risk for GBS.
• Identifying significant quantities of GBS in the urine ofasymptomatic pregnant women.
• Observing newborns for signs of sepsis and testing (culture of the infant's blood or spinal fluid) and treating them promptly.

Guidelines from the CDC also expanded the methods used by laboratories to process samples for GBS screening in order to improve detection. Recommendations include the following:

• The recommended test for prenatal GBS screening is a culture of the vagina and rectum. For this test, a swab of both sites is placed into a selective enrichment broth, which encourages the growth of any GBS present. After the bacteria have been grown and isolated in culture, additional tests will be performed to confirm the presence of GBS. The updated guidelines offer alternative identification tests to enhance detection of the bacteria and to decrease the turnaround time to obtain the final results. In certain instances, molecular tests that detect and identify the genetic material of the bacteria may be employed.
• Current molecular assays performed directly on the vaginal/rectal swab without an enrichment step are not as sensitive as culture. Therefore, these molecular tests are not recommended for routine screening of women in their 35-37th week of pregnancy and who are not yet in labor. Molecular testing of the vaginal/rectal sample is permitted to be used to screen women who are in labor and have no record of prior testing for GBS and who have no obvious risk factors for the disease. Some factors that can increase risk include premature labor, fever, or prolonged rupture of membranes.
• Urine cultures may be used for screening for significant colony counts of GBS.
• Antimicrobial susceptibility testing is not routinely ordered when GBS is isolated; however, testing should be ordered to evaluate alternative treatments if a woman with GBS colonization is severely allergic to penicillin.

Prenatal GBS screening is usually ordered on pregnant women at 35 to 37 weeks gestation per CDC recommendations. A woman can be positive for GBS at one time during her pregnancy and negative at another. That is why it is important to screen a woman late in her pregnancy, closer to the time when she will deliver and could potentially pass the bacteria to her newborn.

Testing should also be performed if a woman goes into labor prior to 35 weeks gestation in order to determine her GBS status.

Antimicrobial susceptibility testing should be ordered whenever a woman is severely allergic to the standard GBS treatment, penicillin or ampicillin, in order to select an effective alternative therapy.

A positive test for GBS in a healthy pregnant woman means that she is currently colonized with GBS and should be given antimicrobial agents intravenously during labor to prevent the transmission of the bacteria to the baby. This treatment is known as intrapartum prophylaxis.

Pregnant women who have a urine culture positive for significant numbers of GBS any time during their pregnancy should receive appropriate intrapartum prophylaxis to prevent disease in their infants.

A negative result means that the woman is not likely to be colonized in the areas tested. No intrapartum prophylaxis is recommended.

Every pregnant woman should know her GBS status before she delivers and discuss with her doctor what prophylaxis will be given during her labor if she is positive for GBS.

CDC recommendations represent contributions from and a consensus amongst a large number of national health organizations, including the American College of Obstetricians and Gynecologists, the American Academy of Pediatrics, the American College of Nurse-Midwives, the American Academy of Family Physicians, and the American Society for Microbiology.