Screening

Screening pregnant women to prevent neonatal group B streptococcal infection

Group B streptococci (GBS) are a leading cause of neonatal sepsis, and are acquired before birth from bacteria colonizing the maternal vagina. Studies have shown that active screening for colonization, together with intrapartum antibiotics given to carriers and at-risk groups, has reduced the incidence of early-onset GBS infection and is the recommended means of prevention in the US.[6][55][56][72] However, this has been disputed in a systematic review that concluded that, although intrapartum antibiotics in GBS-colonized mothers appear to reduce early-onset GBS disease, this may be the result of bias in study design and execution.[57] The American College of Obstetricians and Gynecologists recommends a screening strategy for prevention of early-onset GBS infection.[6]

Since GBS screening and intrapartum antibiotics were introduced, the incidence of early-onset GBS infection has decreased by 80%.[6][23] Resistance to macrolides in GBS isolates has increased over this time.[73] Intrapartum antibiotic treatment has had no effect on the incidence of late-onset disease.

Cases of GBS infection continue to occur despite the introduction of screening, although universal screening results in a greater reduction in early-onset GBS disease than the risk-based approach as the latter misses opportunities for intrapartum antibiotic administration.[11][56] Most cases (if screening strategy is followed) occur in infants born to mothers who had negative GBS cultures on screening (64%), most of whom had an intrapartum risk factor for disease.[74] About 4% to 6% of mothers who test negative at 35 to 37 weeks' gestation will be culture positive at term.[75][76]

Differences in incidence of disease and in interpretation of the same literature has led different countries to adopt different prevention protocols.[2][6][21][62][72][77][78][79][80][81][82][83]

Evaluation of a screening program identified the following areas for improvement to reduce the number of missed opportunities for prevention:[84]

  • Women who deliver preterm with unknown colonization status

  • For women who are allergic to penicillin, cefazolin is the preferred agent for those at low risk for anaphylaxis. Those at high risk from penicillin allergy receive either clindamycin (if isolate is sensitive and has no inducible resistance on laboratory testing) or a glycopeptide (e.g., vancomycin) (if resistance or inducible resistance is present, or sensitivities are unknown)

  • Women with false-negative screening results (by improved management processes for screening).

How screening is performed

In the US, vaginal and rectal cultures are obtained at 36 to 37 weeks' gestation for all pregnant women except those with GBS bacteriuria during the current pregnancy, or those who have had a baby with GBS disease previously.[6] [85]

A single swab is taken from the lower vagina and rectum and cultured in enrichment broth and plated on selective media.[6][86] The sensitivity of cultures is greatest when samples are kept at 39.2°F (4°C) prior to culture and processed within 24 hours of collection.[86] Laboratories should process samples by incubating first in an appropriate enrichment broth medium to optimize sensitivity of subsequent culture results.[86] Self-sampling has been found to be an accurate and acceptable alternative to clinician sampling.[87]

Sensitivity testing should be performed on prenatal GBS isolates from penicillin-allergic women at high risk of anaphylaxis to penicillin or cephalosporins. Testing for inducible clindamycin resistance should be performed on isolates from penicillin-allergic women at high risk of anaphylaxis that are sensitive to clindamycin or resistant to erythromycin.[6][86] 

Clinicians should inform the laboratory when sending urine samples from pregnant women. Urine results positive for GBS at ≥10⁴ colony forming units in either pure or mixed cultures should be reported.[6]

Response to a positive result

Intrapartum antibiotics are given to all women colonized with GBS. In addition, antibiotics are indicated for women with:[6][55][63]

  • Previous infant with invasive GBS infection

  • GBS bacteriuria or infection during current pregnancy

  • Unknown GBS status (culture not done, incomplete, or results unknown) and any of the following: intrapartum temperature ≥100.4°F (≥38°C); amniotic membrane rupture >18 hours; or delivery at <37 weeks' gestation.

Response to a negative result

Antibiotics are only indicated for women with:[6][55][63]

  • Previous infant with invasive GBS infection

  • GBS colonization, bacteriuria, or infection during current pregnancy

  • Unknown GBS status (culture not done, incomplete, or results unknown) and any of the following: intrapartum temperature ≥100.4°F (≥38°C); amniotic membrane rupture >18 hours; or delivery at <37 weeks' gestation.

Response if screening not done

Antibiotics are indicated for women with:[6][55][63]

  • Previous infant with invasive GBS infection

  • GBS colonization, bacteriuria, or infection during current pregnancy

  • Unknown GBS status (culture not done, incomplete, or results unknown) and any of the following: intrapartum temperature ≥100.4°F (≥38°C); amniotic membrane rupture >18 hours; or delivery at <37 weeks' gestation.

Screening for GBS carriage at onset of labor

The difficulties associated with culture-based screening, and the potential for women who screen negative at 36 to 37 weeks' gestation to be positive at delivery (approximately 4%) and for women who screen positive but are no longer carrying at delivery (approximately 13%), have led several investigators to look for more rapid diagnostic tests that can be carried out at the onset of labor.[75] Polymerase chain reaction (PCR) testing is one such possible method that has a high sensitivity and specificity and can be carried out in 30 to 45 minutes.[88] However, none of the available tests are sufficiently accurate and none are cost effective.[86] In addition the logistics of running a PCR assay 24 hours a day, 7 days a week in real clinical practice prohibit its use in many settings.[79] Point-of-care tests are also under development, but none are currently in routine use or endorsed by any of the national guidelines. The use of any such product would have to be evaluated and endorsed locally.

Screening for GBS at the onset of preterm labor or at the time of preterm premature rupture of membranes should be undertaken in cases where GBS status has not been assessed in the preceding 5 weeks. Antibiotic prophylaxis should be commenced and subsequently discontinued if negative screening results become available or if the mother does not enter true labor and the membranes remain intact. Screening should be repeated between 36 and 37 weeks' gestation if the mother has not yet delivered.[6]

US guidelines endorse the use of nucleic acid amplification testing (NAAT) in this setting where it is available. NAAT can be used for intrapartum testing of vaginal-rectal samples from women with unknown GBS colonization status and no intrapartum risk factors (temperature >100.4°F [>38°C] or rupture of amniotic membranes >18 hours) at the time of testing and who are delivering at term. Antibiotic prophylaxis is recommended for women with positive intrapartum NAAT results or if an intrapartum risk factor subsequently develops (regardless of the NAAT result).[6][86] 

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