At a Glance
Why Get Tested?
To screen a pregnant woman for the presence of group B streptococcus (GBS) to determine the risk that she will pass the bacteria on to her newborn, possibly resulting in a serious infection
When to Get Tested?
When a pregnant woman is between 35 and 37 weeks of gestation, has preterm labor, or preterm premature rupture of membranes
A culture swab obtained from the vagina and rectum of a pregnant woman; a urine sample collected anytime during pregnancy may be screened for GBS
Test Preparation Needed?
The Test Sample
What is being tested?
Group B strep screening identifies the presence of the bacteria Streptococcus agalactiae, also known as group B beta-hemolytic streptococcus and group B streptococcus (GBS), in the vaginal/rectal area of a pregnant woman. GBS is a common bacterium that colonizes the gastrointestinal tract and genital tract. It rarely causes problems in healthy adults but can cause infections and serious illness in:
- Newborns; this is the area of greatest concern. GBS can be passed from mother to child before or during delivery, causing early-onset GBS disease that appears within hours to days of birth. Infants can also contract late-onset GBS from 7 days to several months after birth. GBS can cause serious infections such as pneumonia, sepsis, and meningitis. According to the Centers for Disease Control and Prevention, it is the most common cause of life-threatening infections in newborns.
- Women who have given birth (postpartum) or who have undergone other obstetric or gynecological procedures; GBS may cause infections of the lining of the uterus (endometritis).
The number of infants with GBS disease has decreased significantly in recent years because of a concerted effort by health care providers to screen pregnant women for GBS late in their pregnancy and, when they are positive for GBS, to treat them during delivery. This prevents or greatly decreases the risk of passing the bacteria to the newborn. Nevertheless, GBS disease remains the primary cause of early-onset sepsis, a serious and life-threatening infection in newborns. Currently there is no vaccine available to prevent GBS, and treating all pregnant women with antibiotics is not practical, so screening for GBS continues to be the best means for preventing GBS disease in newborns.
The primary test for GBS is a culture. For this test, the sample is placed into or on appropriate nutrient media, which encourages the growth of any GBS present, which allows further testing and identification. After the bacteria have been grown and isolated in culture, additional biochemical tests can be performed to confirm the presence of GBS. In certain instances, molecular tests that detect and identify the genetic material of the bacteria may be employed.
How is the sample collected for testing?
For screening pregnant women, a swab is typically obtained from the vagina and rectum. In addition, urine collected during pregnancy may be cultured for significant numbers of GBS.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
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Comer, Yun, H. and Hamza, H. (Updated 2010 February 11). Bacterial Infections and Pregnancy. eMedicine [On-line information] Available online at http://emedicine.medscape.com/article/235054-overview through http://emedicine.medscape.com. Accessed October 2010.
Mayo Clinic Staff (2008 December 5). Group B strep MayoClinic.com [On-line information] Available online at http://www.mayoclinic.com/health/group-b-strep/DS01107/METHOD=print through http://www.mayoclinic.com. Accessed October 2010.
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(2010 March). Group B Strep Infection. March of Dimes Fact Sheet [On-line information] Available online at http://www.marchofdimes.com/professionals/14332_1205.asp through http://www.marchofdimes.com. Accessed October 2010.
Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, 12th Edition: Mosby Elsevier Press, St. Louis, MO. Pp 265-279.
(November 19, 2010) Centers for Disease Control and Prevention, Morbidity and Mortality Weekly Report, Prevention of Perinatal Group B Streptococcal Disease, 59(RR10);1-32. Available at http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5910a1.htm?s_cid=rr5910a1_w through http://www.cdc.gov. Accessed November 2010.
Centers for Disease Control and Prevention. Group B Strep (GBS). Available online at http://www.cdc.gov/groupbstrep/index.html through http://www.cdc.gov. Accessed November 2010.