Group B Strep (GBS) Test
Reviewed
Also called: Group B Streptococcus, Group B Beta-Hemolytic Streptococcus, Streptococcus agalactiae, Beta-hemolytic strep culture
A group B strep test checks for GBS bacteria in the body. Many healthy people carry GBS without symptoms, but it can cause serious infections in newborns, older adults, and people with weakened immune systems. In pregnant women, testing is done between 36 and 38 weeks to guide antibiotic use during labor and prevent transmission to the baby.
Specimen: Swab from vagina and rectum, or urine (clean-catch), or blood (heel stick in newborns), or cerebrospinal fluid (lumbar puncture in newborns)
Why this test may be ordered
- Routine prenatal screening between 36 and 38 weeks of pregnancy
- If labor begins before 36 weeks
- Newborns with signs of infection such as fever, trouble feeding, trouble breathing, lack of energy, irritability, bluish skin, unstable blood pressure, or kidney problems
- Symptoms of GBS disease in non-pregnant adults: fever, chills, low alertness, cough, rapid breathing, chest pain, red/swollen/painful skin bumps with pus, bone/joint pain with swelling or stiffness
Preparation and collection
No special preparations needed.
For pregnant women: a swab of cells and fluids from the vagina and rectum, or a clean-catch urine sample. For newborns: a blood sample from the heel or a lumbar puncture (spinal tap) to collect cerebrospinal fluid.
There is no risk from a swab or urine test. Newborns may have slight pain or bruising after a blood test, which goes away quickly. A spinal tap may cause some pain, and there is a small risk of infection or bleeding.
Understanding results
Your healthcare provider will interpret your test results. The following patterns are based on the information provided by MedlinePlus.
- GBS detected (positive): You have GBS bacteria in your body. If you are pregnant, you will be given antibiotics intravenously during labor (at least four hours before delivery) to prevent passing the bacteria to your baby. Antibiotics earlier in pregnancy are not effective because the bacteria can grow back quickly. Limitation: This result does not mean you are sick; many healthy people carry GBS without symptoms. Taking antibiotics by IV during labor is more effective than by mouth. Next step: Prompt
- GBS not detected (negative): No GBS bacteria were found in the sample. This is a routine finding. Limitation: Testing is typically done between 36 and 38 weeks; if you go into labor earlier, you may need to be tested at that time. Next step: Routine
- GBS bacteria can grow back quickly after antibiotics, so treatment earlier in pregnancy is not effective.
- In newborns, early-onset GBS disease symptoms usually appear on the day of birth; late-onset can appear between 7 and 89 days of life, and the source is unknown.
- If your provider suspects a GBS infection, treatment may begin before test results are available because GBS can cause serious illness or death.
What to do next
If you are pregnant and test positive, you will receive IV antibiotics during labor. If your baby tests positive, they will be treated with antibiotics. Discuss any questions about your or your baby's results with your healthcare provider.
- What does a positive GBS test mean for my labor and delivery?
- If my baby tests positive, what treatment will be given?
- Do I need antibiotics if I have a planned C-section?