Group B Strep
/Today’s episode covers the ACOG Committee Opinion on Group B Streptococcus, also known as GBS or Streptococcus agalactiae.
The OBG Project has a phenomenal summary of these new guidelines; we encourage you to check out their outline which was hugely helpful in preparing the podcast for today! And be sure to check out how you can get their premium product, OBG First, absolutely FREE if you’re a chief resident. Check out the sidebar to learn more!
Get to know Group B Strep
Streptococcus agalactiae or GBS is a common flora of the vagina and rectum of women, with a prevalence of colonization between 10-30%. In newborns, two types of GBS-related disease exist:
Late Onset Disease occurs between 7 days and 2-3 months after birth, and is characterized by sepsis, more commonly meningitis, and organ/soft tissue infection.
More commonly acquired by horizontal transmission from the mother; can also be acquired from hospital sources or individuals in the community.
Curious: late onset cases caused by placental ingestion with improper processing.
Early Onset Disease is present within 7 days of birth.
Occurs secondary to vertical transmission; fetal/neonatal aspiration during labor process; or both.
Manifestations of disease occur most likely within 12-48 hours after birth.
Characterized by sepsis, pneumonia, and less commonly meningitis.
The most common cause of early-onset neonatal sepsis.
72% of cases of Early Onset Disease occur in term newborns; however, mortality is markedly higher in premature infants (19.2% in premies vs 2.1% at term), and premature infants are more likely to require higher levels of intensive care intervention.
When treating GBS in labor, this is what we are primarily aiming to prevent!
Risk factors for GBS Early Onset Disease:
Gestational age < 37 weeks
Very low birth weight
Prolonged rupture of membranes > 18 hours
Intraamniotic infection
Young maternal age
Black race
Heavy vaginal-rectal colonization or GBS bacteriuria (proxy for heavy colonization)
Previous newborn affected by GBS early onset disease
The Screening Recommendations
Screening in the USA is universal, meaning this should be a standard part of prenatal care.
ACOG recommends universal screening for vaginal-rectal colonization between 36’0 - 37’6.
These results are considered “valid” for 5 weeks — thus the majority of women will be captured with screening at 36’0 (assuming inductions for most women at 41’0).
This is also recommended for women planning to deliver by cesarean.
What about GBS bacteriuria?
If GBS bacteriuria is present in any amount at any time during pregnancy, this is considered a positive result (proxy for heavy colonization), and thus repeat screening is not necessary.
If asymptomatic GBS bacteriuria is present at >10^5 CFU/mL, treatment should be considered as you would for any other form of ASB.
If at less than 10^5 CFU, no correlation has been found between treatment of this lower-level bacteriuria and improved maternal or neonatal outcomes; however, it should still be noted that this patient would be considered GBS positive.
Screening may also occur at the time of admission for preterm labor and prelabor premature rupture of membranes (PPROM).
In the case of prematurity, treatment should begin while awaiting screen results.
If the screen is positive and preterm labor resolves, the colonization should be considered positive for the remainder of the pregnancy.
If the screen is negative, re-screening should be performed after 5 weeks.
The gold standard for screening is obtaining a vaginal-rectal culture.
Recently, nucleic acid amplification tests (NAAT) have also demonstrated promise in obtaining accurate, quick results.
Most accurate tests that perform similar to culture do require some culture time prior to the NAAT test performance; results generally aren’t available before 18-24 hours after screening occurs.
NAAT tests that offer results in a shorter time period often have lower accuracy.
NAAT tests also do not have the ability to test antibiotic resistance, which is important in the case of a patient with known penicillin allergy.
If your patient has known penicillin allergy, the laboratory should be alerted so sensitivity testing can be performed.
Given this delay and with someone moving quickly in labor or preterm labor with no known GBS status, risk factor-based screening may also be employed to decide if treatment should be pursued in the absence of a screening result.
If any of the risk factors are present, treatment should be pursued:
Prematurity or PPROM (< 36 weeks 6 days)
History of a prior newborn affected by GBS disease
Amniotic membrane rupture > 18 hours duration
Presence of intrapartum fever > 100.4F (38 C)
If known GBS positive result in a previous pregnancy (may engage in shared decision making in this clinical scenario).
Intrapartum Antibiotic Treatment
The gold standard for the treatment of GBS colonization intrapartum to reduce risk to neonates for early-onset disease is penicillin G.
Dosed at a loading dose of 5 million units loading, then 2.5-3 million units IV every 4 hours until delivery.
Ampicillin is an acceptable alternative, with a loading dose of 2g IV followed by 1g IV every 4 hours.
PCN is preferred as it has a narrower, more targeted spectrum of activity and lower likelihood of inducing resistance in other organisms.
For this reason, identification and history of penicillin allergy is super important to flush out in prenatal care!
80-90% of persons with reported PCN allergy are not truly allergic.
Pruritic rash, urticaria (hives), immediate flushing, angioedema, respiratory distress, or anaphylaxis after PCN or cephalosporin administration is considered a high risk for true allergy.
Even with high risk persons, if PCN allergy testing has not been performed, it is recommended to do this — even in pregnancy!
When PCN allergy testing has not been performed, the allergy should be classified as “low” versus “high risk,” based on the symptoms just described.
If “low risk,” first generation cephalosporins such as cefazolin are recommended.
Cefazolin has very low cross reactivity with penicillin for a 1st gen cephalosporin, and GBS remains highly susceptible to it.
Dosed at 2g IV load, followed by 1g IV q8h until delivery.
If “high risk,” or an unknown risk with no allergy testing, susceptibility testing should be performed by the laboratory with screening.
Options for therapy will include clindamycin or vancomycin.
Clindamycin should only be utilized if culture results have shown susceptibilit.
Resistance to clindamycin in GBS is approximately 20%.
Dosing: 900mg q8h until delivery
Vancomycin has good activity against GBS.
High risk in creating resistant organisms (i.e., VRE) with widespread use, and thus every effort should be made to rule out penicillin allergy before its use.
Dosing: 20mg/kg IV q8h, with maximum of 2g per single dose.
How long does it take the antibiotics to work?
All antibiotics used for GBS prophylaxis are time-dependent with respect to their ability to lower microbial load.
Studies done with PCN or ampicillin prophylaxis demonstrate that 4 or more hours pf prophylaxis is preferable, though 2 hours has been shown to reduce GBS count and decrease neonatal sepsis.
That said, obstetric intervention should not be delayed solely to provide 4 hours of antibiotic administration., when it is indicated.
Examples of such interventions that should not otherwise be delayed include oxytocin administration, AROM, or cesarean.
That said, if interventions are not immediately indicated, the benefits of increased antibiotic exposure to reduce GBS exposure should be considered.