Also Known As
Group B Beta-hemolytic Streptococcus
Group B Streptococcus
Group B Strep
GBS
Strep Group B
Streptococcus agalactiae
Beta-hemolytic Strep Culture
Formal Name
Prenatal Group B Streptococcus Screening
This article was last reviewed on
This article waslast modified on November 5, 2017.
At a Glance
Why Get Tested?

To screen a pregnant woman for the presence of group B streptococcus (GBS) in her vagina or rectum to determine the risk that she will pass the bacteria on to her newborn during labor and delivery, possibly resulting in a serious infection in her newborn called early-onset GBS

When To Get Tested?

When a woman is between 35 and 37 weeks of pregnancy or has preterm labor or preterm premature rupture of membranes

Sample Required?

A swab obtained from the vagina and rectum of a pregnant woman; a urine sample collected anytime during pregnancy may be used to detect significant numbers of GBS.

Test Preparation Needed?

None

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The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

Group B strep (GBS) is the common name for the bacterium Streptococcus agalactiae that can be present (colonizing) in the digestive tract and genital tract. It rarely causes symptoms or problems in healthy adults but can cause infections and serious illness in newborns who become infected before or during labor and delivery. GBS screening identifies the presence of the bacteria in the vaginal/rectal area of a pregnant woman.

GBS can cause early-onset GBS disease that occurs within the first week after birth. Signs and symptoms in newborns include fever, difficulty with feeding and breathing, irritability or lethargy, and a blue tint to the skin. 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.

Approximately 25% of pregnant women carry group B strep in their rectum or vagina. However, the number of infants with GBS disease has decreased significantly in recent years because of a concerted effort by healthcare practitioners to screen pregnant women for GBS late in their pregnancy and, when they are positive for GBS, to treat them with intravenous antibiotics (usually penicillin or ampicillin) during labor. 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. Screening for GBS and appropriate treatment continues to be the best means for preventing GBS disease in newborns.

How is the sample collected for testing?

For screening pregnant women, a swab is typically obtained from the vagina and rectum. Urine collected during pregnancy may be cultured for significant numbers of GBS.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Accordion Title
Common Questions
  • How is it used?

    A prenatal screening test for group B strep (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 called early-onset GBS.

    The Centers for Disease Control and Prevention (CDC) recommend:

    • Screening all pregnant women for the presence of GBS in the vagina and rectum between 35 and 37 weeks of pregnancy unless GBS has already been detected in a urine sample during the pregnancy or the woman has had a previous infant with invasive GBS disease. When GBS is detected during screening, the woman should be treated with intravenous (IV) antibiotics during labor and delivery.
    • If a woman has already had GBS detected in a urine sample earlier in her pregnancy or had a previous newborn with GBS, she should automatically receive IV antibiotics during delivery.
    • Pregnant women who have not yet been tested for GBS and have preterm labor or whose membranes have ruptured more than 18 hours previously or who have a fever during labor should receive IV antibiotics.

    Healthcare practitioners check newborns soon after birth for signs and symptoms of GBS and sepsis, and if present, perform additional testing (e.g., culture of the infant's blood or spinal fluid) and treating newborns promptly if necessary.

    GBS testing may 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. If bacteria grow in the broth, additional tests are done to confirm the presence of GBS.
    • In certain instances, molecular tests that detect and identify the genetic material of the bacteria directly in vaginal and rectal swabs may be used. Direct testing without broth enrichment is not as sensitive as culture, so these tests are not recommended for routine screening of women in their 35th to 37th week of pregnancy and who are not yet in labor. They may 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 numbers of GBS.
    • Antimicrobial susceptibility testing is not routinely done since GBS is routinely susceptible to ampicillin. However, testing should be performed to determine effective antibiotic therapy for women who are positive for GBS and severely allergic to penicillin and related antibiotics.
  • When is it ordered?

    Group B strep screening is usually ordered when a woman is 35 to 37 weeks pregnant. 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 when a woman goes into labor prior to 35 weeks of pregnancy to determine her GBS status.

  • What does the test result mean?

    A positive test for GBS in a healthy pregnant woman means that the bacteria are currently present in her vagina and/or rectum and she is at risk of passing the bacteria to her infant. She should be given antibiotics intravenously during labor to prevent spreading the bacteria to her 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 have GBS in the areas tested. No intrapartum prophylaxis is recommended unless she has other risk factors.

  • Is there anything else I should know?

    Every pregnant woman should know her GBS status before she delivers and discuss with her healthcare practitioner what antibiotic treatment 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.

  • Is it possible to prevent a group B strep (GBS) infection in newborns?

    Administering antibiotics during labor to those who are colonized with GBS has greatly decreased the incidence of babies born with early-onset GBS infection. A GBS vaccine is not currently available.

  • Is GBS considered a sexually transmitted disease (STD)?
    No. It is a common bacterium that colonizes both men and women and is not considered an STD.
  • Should every woman be treated with antibiotics for GBS during delivery or during pregnancy?

    No. Since about 75% of pregnant women are not colonized by GBS at the time of delivery, it makes the most sense to treat the 25% who are. The numbers of GBS present can vary throughout a woman's pregnancy, so treatment earlier during a pregnancy does not ensure that GBS is not present at the time of delivery.

  • Can babies get GBS infections more than 7 days after they are born?

    Infants can also contract late-onset GBS infection from 7 days to several months after birth. Late-onset disease is not preventable by administering antibiotics during labor since the bacteria are not acquired during pregnancy or during the birth process.

  • What is the difference between group A and group B streptococcus?

    Group A streptococcus (GAS) and group B streptococcus (GBS) are similar types of bacteria, but they cause different types of infections. Group A strep is not usually part of the normal bacterial flora. It is the bacteria that causes "strep throat" that is detected using a rapid strep test or a throat culture. Group A strep may also cause severe skin and wound infections. Group B strep can be part of the normal bacteria found in the throat, vaginal tract, and digestive tract. GBS causes infections in newborns and in adults with weakened immune systems.

  • Does GBS cause serious infections in adults?

    Yes. Although GBS usually does not cause illness in healthy adults, it may cause infections of the lining of the uterus (endometritis) in women who have given birth (postpartum) or who have undergone other obstetric or gynecological procedures. GBS also causes wound infections and serious blood and central nervous system infections in adults, especially in those who have diabetes.

    GBS can also cause skin infections, pneumonia, septicemia and urinary tract infections (UTI) in the elderly and those with underlying health conditions and/or weakened immune systems, congestive heart failure, vascular disease, liver disease, heart disease, AIDS, or cancer.

    General screening of asymptomatic people, as it is discussed in this article, is not performed in these populations. Cultures of the affected areas of the body are done only if signs and symptoms of infection are present.

View Sources

Sources Used in Current Review

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(2016 May 23 Updated). Group B Strep (GBS), Prevention in Newborns Centers for Disease Control and Prevention. Available online at https://www.cdc.gov/groupbstrep/about/prevention.html. Accessed on 3/19/17.

Burd, I. and Zieve, D. (2016 January 20 Updated). Group B streptococcus – pregnancy. MedlinePlus Medical Encyclopedia. Available online at https://medlineplus.gov/ency/patientinstructions/000511.htm. Accessed on 3/19/17.

(© 1995–2017). Group B Streptococcus (Streptococcus agalactiae) Culture. Mayo Clinic Mayo Medical Laboratories. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/87346. Accessed on 3/19/17.

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Woods, C. and Levy, C. (2016 October 24 Updated). Streptococcus Group B Infections. Medscape Drugs and Diseases. Available online at http://emedicine.medscape.com/article/229091-overview Accessed on 3/19/17.

Lee, K. and Zieve, D. (2014 December 12 Updated). Group B streptococcal septicemia of the newborn. MedlinePlus Medical Encyclopedia. Available online at https://medlineplus.gov/ency/article/001366.htm. Accessed on 3/19/17.

Bidgani, S. et. al. (2016 March). Comparison of group B streptococci colonization in vaginal and rectal specimens by culture method and polymerase chain reaction technique. J Chin Med Assoc. 2016 Mar;79(3):141-5. Available online at https://www.ncbi.nlm.nih.gov/pubmed/26803202. Accessed on 3/19/17.

Sources Used in Previous Reviews

Woods, C. and Levy, C. (Updated 2010 May 17). Streptococcus Group B Infections. eMedicine [On-line information] Available online at http://emedicine.medscape.com/article/229091-overview. Accessed October 2010.

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. 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. Accessed October 2010.

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