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D.B. is a 27 year old gravida 2, para 1, who was in the office recently for her 36 week check. While examining her, I obtained a swab for a Group B Strep screening culture.
I used a standard Culturette swab, inserted into the lower vagina without lubrication. Having sampled the lower vagina, the swab was now lubricated with vaginal secretions, which allowed for a relatively easy insertion of the same swab through the anus and into the rectum, again without external lubrication. I placed the swab in transport media and sent it for Group B Strep culture.
Early onset Group B strep infections are relatively uncommon, but can be devastating to the newborn. Prior to screening for GBBS (Group B Beta Hemolytic Streptococcus), this condition was found in about 1.5/1000 newborns.
GBBS can be cultured in about 20% of women in early pregnancy. This is, for the most part, an asymptomatic infection, although GBBS can certainly cause such problems as symptomatic urinary tract infections, chorioamnionitis, bacteremia and sepsis. But the larger problem arises shortly after the baby is born, at which time there has been an opportunity for ascending exposure of the fetus to the maternal GBBS. The majority of newborns successfully fight off the GBBS exposure and don’t get sick. For a few (about 1.5 in a thousand), they become desperately ill and may not survive. Some of the survivors will have permanent disabilities. The point is that early onset GBBS infection in the newborn, while not common, can be catastrophic and we should try to avoid it.
A number of strategies to prevent early onset GBBS disease in newborns have been proposed. Some are reasonably effective and some are not. Screening all women for the presence of GBBS early in pregnancy will have about a 20% positive yield. If you treat all of them with antibiotics and then rescreen everyone late in pregnancy, you will again find about 20% positives for GBBS. Some of those positives will be the same women you identified early in pregnancy, and some will be different women who were originally negative. If you don’t treat the women early in pregnancy, but still reculture late in pregnancy, you have the same results…still 20% positive, including some who were positive early, and others who were not. And some of the early positives will no longer be positive. OK, so we don’t screen early, we screen late in pregnancy, ideally between the 35th and 37 week of pregnancy.
Once we identify a positive test, then we don’t do anything other than informing the woman that she’s tested positive, until she comes into labor or ruptures her membranes. At that point, we start antibiotics. At my hospital, we give a loading dose of 2 grams of IV ampicillin, followed by 1 gram IV ever 4 hours. There are other antibiotic regimens that can also give good results. If she is penicillin allergic, then we usually give clindamycin, although again, there are a number of reasonable alternatives.
Once loaded with antibiotics, we see an improvement in the subsequent incidence of early onset GBBS disease. The maximum benefit occurs after 4 hours of IV therapy. So once we start the antibiotic, we’re in no hurry for the patient to deliver until after 4 hours has passed. If the patient is in extremely active labor and destined to deliver before we get the full 4 hours of antibiotic in, we usually don’t actively intervene to stop labor, because that intervention raises additional risks for the fetus.
Using this strategy, we can lower the risk of early onset GBBS from 1.5/1000 to about 0.3/thousand, or roughly an 80% reduction in risk. That’s very good, not perfect, but very good.
Of course, there are some additional complexities that I should comment on:
1. If the patient has already tested positive for GBBS (for example, in a urine culture) during this pregnancy, then we don’t reculture, we just treat during labor.
2. If the patient is in preterm labor or has ruptured their membranes prematurely, prior to our testing for GBBS at 36 weeks, then we go ahead and treat them for GBBS anyway. Some authorities recommend culturing and then stopping the antibiotics once you have the culture results back, but others are reluctant to stop the antibiotics in this setting.
3. If the patient is undergoing a cesarean section and has not ruptured her membranes, then the antibiotics for GBBS are not necessary. Although we usually give some sort of antibiotic prophylaxis to women about to undergo cesarean section, to prevent maternal infection.
4. If the woman has had a previous infant with GBBS infection, we’ll automatically give antibiotic prophylaxis in labor with the current infant, without regard to screening status.
5. If the woman develops a fever in labor during labor, we usually will begin antibiotics to cover presumed chorioamnionitis and the antibiotic selection will generally include coverage for GBBS, without regard to screening status.
6. This culture based screening protocol is not the only strategy used to address the GBBS issue. The primary variation is the risk-based method, in which women deemed to be at higher risk for GBBS are given antibiotic prophylaxis. This strategy avoids the costs involved in culturing everyone and tracking the results, but most authorities agree it is not as effective in reducing the risk of early onset GBBS disease among newborns.
