Prenatal Group B Strep Screening

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Also known as: Streptococcus agalactiae; Group B beta-hemolytic streptococcus; Group B streptococcus; Group B strep; GBS; Strep group B; Beta-hemolytic strep culture
Formal name: Prenatal Group B streptococcus Screening
Related tests: Urine Culture; Gram Stain; Susceptibility Testing

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

Sample Required?

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?

None

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.

The Test

How is it used?

Screening tests for group B streptococcus (GBS) are used to detect the presence of the bacteria in the genital, urinary, and/or gastrointestinal tract of pregnant women. About 25% of 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.

Screening is performed following recommendations issued by the Centers for Disease Control and Prevention (CDC). New recommendations were released on November 19, 2010 in a report that presents updated guidelines for the prevention of perinatal group B streptococcal (GBS) disease. It outlines the procedures for the universal screening of pregnant women at 35–37 weeks gestation to detect GBS colonization. It also details 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.
  • Screening for and identifying significant quantities of GBS in the urine of asymptomatic pregnant women.
  • Observing newborns for signs of sepsis and testing 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 primary means of testing is vaginal/rectal cultures because it is the most sensitive. Some changes in the guidelines in how these are performed are intended to enhance detection of the bacteria and to speed up the identification process.
  • Molecular testing that detects the genetic material of the bacteria 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. However, since molecular testing is not as sensitive as culture, it is not recommended for routine screening of women in their 35-37th week of pregnancy and who are not yet in labor. Molecular tests may be used as a follow-up test to help identify GBS bacteria grown in certain types of cultures.
  • 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 allergic to penicillin.

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When is it ordered?

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 possibly spread the bacteria to her newborn.

Testing should be done on a woman who presents in labor prior to 35 weeks gestation to determine her GBS status.

Antimicrobial susceptibility testing should be ordered whenever a woman is allergic to the standard GBS treatment, penicillin.

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What does the test result mean?

A positive test for GBS in a healthy pregnant woman means that she is currently colonized with GBS. A negative result means that the woman is not likely to be colonized in the areas tested. A positive GBS in a symptomatic pregnant woman may be due to a urine, uterine, or other GBS infection.

If the test is positive, the woman will be given antimicrobial drugs intravenously during labor, and also if she has a symptomatic infection.

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Is there anything else I should know?

Since culture results take 2-3 days to complete, treatment is usually started on women who present with premature labor or ruptured membranes and on those whose GBS status is unknown, before test results are available.

CDC recommendations represent contributions from and a consensus amongst a large number of national health organizations. They include 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.

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Common Questions

1.  Is it possible to prevent a Group B strep (GBS) infection?

Prevention of the transmission from mother to newborn has been successful. Researchers have been exploring the possibility of a GBS vaccine, but one is not currently available.

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2.  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.

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3.  Should every woman be treated with antibiotics for GBS during delivery or during pregnancy?

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. Since GBS colonization comes and goes, treatment during a pregnancy would not ensure that GBS is not present at the time of delivery.

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4.  What is the difference between group A and group B streptococcus?

They are related but are two different types of bacteria. Group A streptococcus (GAS) is commonly associated with causing "strep throat." Strep throat is detected using a strep test or a throat culture.

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5.  Does GBS cause serious infections only in pregnant women and newborns?

GBS can also affect the elderly and those with underlying health conditions and/or weakened immune systems, such as those with diabetes, congestive heart failure;, vascular disease, liver disease, heart disease, AIDS, or cancer. In these people, GBS can cause skin infections, pneumonia, septicemia and urinary tract infections (UTI), among others. However, general screening on 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 are present.

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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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