Group B streptococcal infection screening

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor(s)-in-Chief: Rim Halaby, M.D. [3]

Overview

The Center of Disease Control and Prevention (CDC)'s screening guidelines for Group B streptococcal (GBS) infection recommend universal culture-based screening for all pregnant women for vaginal and rectal GBS colonization in order to determine which women should receive intrapartum GBS chemoprophylaxis. CDC recommended that women with unknown GBS colonization status at the time of delivery be managed according to the presence of intrapartum risk factors. CDC's guidelines recommend screening for vaginal and rectal GBS colonization at 35-37 weeks' gestation. Swabbing both the lower vagina and rectum (through the anal sphincter) increases the culture yield substantially compared with sampling the cervix or the vagina without also swabbing the rectum. Routine screening for asymptomatic bacteriuria is recommended in pregnant women, and laboratories should screen urine culture specimens for the presence of GBS in concentrations of 104 colony-forming units (cfu)/ml or greater.

Screening

Indications

CDC's guidelines recommend universal culture-based screening for all pregnant women for vaginal and rectal GBS colonization (class A, level of evidence II) in order to determine which women should receive intrapartum GBS chemoprophylaxis. CDC recommended that women with unknown GBS colonization status at the time of delivery be managed according to the presence of intrapartum risk factors.

Timing

CDC's guidelines recommend screening for vaginal and rectal GBS colonization at 35-37 weeks' gestation.

Because GBS colonization status can change over the course of a pregnancy, the timing of specimen collection for determination of colonization status is important. Because colonization can be transient, colonization early in pregnancy is not predictive of early-onset GBS disease. Late third trimester colonization status has been used as a proxy for intrapartum colonization. The negative predictive value of GBS cultures performed ≤5 weeks before delivery is 95%-98%; however, the clinical utility decreases when a prenatal culture is performed more than 5 weeks before delivery because the negative predictive value declines.

Screening in Preterm Labor

Shown below is an algorithm for screening for group B streptococcal (GBS) colonization and use of intrapartum prophylaxis for women with preterm labor (gestational age <37 weeks) among whom no previous screening was performed based on the 2010 revised CDC guidelines.

The patient is admitted with signs and symptoms of preterm labor*

Obtain vaginal and rectal swab for GBS culture† and start GBS prophylaxis

Is the patient entering true labor?

Yes

No

Continue GBS prophylaxis until delivery**

Discontinue GBS prophylaxis

What is the result of the GBS culture?

Positive

Not available prior to labor onset and patient is still preterm

Negative

Administer GBS prophylaxis at onset of true labor

Do not administer GBS prophylaxis at onset of true labor†† Repeat vaginal and rectal culture if patient reaches 35-37 weeks of gestation and has not yet delivered§

* At <37 weeks and 0 days' gestation.

† If patient has undergone vaginal-rectal GBS culture within the preceding 5 weeks, the results of that culture should guide management. GBS-colonized women should receive intrapartum antibiotic prophylaxis. No antibiotics are indicated for GBS prophylaxis if a vaginal-rectal screen within 5 weeks was negative.

¶ Patient should be regularly assessed for progression to true labor; if the patient is considered not to be in true labor, discontinue GBS prophylaxis.

** If GBS culture results become available prior to delivery and are negative, then discontinue GBS prophylaxis.

†† Unless subsequent GBS culture prior to delivery is positive.

§ A negative GBS screen is considered valid for 5 weeks. If a patient with a history of PTL is re-admitted with signs and symptoms of PTL and had a negative GBS screen >5 weeks prior, she should be rescreened and managed according to this algorithm at that time.

Screening in Preterm Premature Rupture of Membrane

Shown below is an algorithm for screening for group B streptococcal (GBS) colonization and use of intrapartum prophylaxis for women with preterm (gestational age <37 weeks) premature rupture of membranes (pPROM) among whom no previous screening was performed based on the 2010 revised CDC guidelines.

The patient is admitted with signs and symptoms of preterm* premature rupture of membrane

Obtain vaginal and rectal swab for GBS culture† and start antibiotics for latency or GBS prophylaxis

Is the patient entering true labor?

Yes

No

Continue antibiotics until delivery

Continue antibiotics per standard of care if receiving for latency OR Continue antibiotics for 48 hours** if recieving GBS prophylaxis

What is the result of the GBS culture?

Positive

Not available prior to labor onset

Negative

Administer GBS prophylaxis at onset of true labor

Do not administer GBS prophylaxis at onset of true labor†† Repeat vaginal and rectal culture if patient reaches 35-37 weeks of gestation and has not yet delivered§

* At <37 weeks and 0 days' gestation.

† If patient has undergone vaginal-rectal GBS culture within the preceding 5 weeks, the results of that culture should guide management. GBS-colonized women should receive intrapartum antibiotic prophylaxis. No antibiotics are indicated for GBS prophylaxis if a vaginal-rectal screen within 5 weeks was negative.

§ Antibiotics given for latency in the setting of pPROM that include ampicillin 2 g intravenously (IV) once, followed by 1 g IV every 6 hours for at least 48 hours are adequate for GBS prophylaxis. If other regimens are used, GBS prophylaxis should be initiated in addition.

** GBS prophylaxis should be discontinued at 48 hours for women with pPROM who are not in labor. If results from a GBS screen performed on admission become available during the 48-hour period and are negative, GBS prophylaxis should be discontinued at that time.

†† Unless subsequent GBS culture prior to delivery is positive.

§ A negative GBS screen is considered valid for 5 weeks. If a patient with pPROM is entering labor and had a negative GBS screen >5 weeks prior, she should be rescreened and managed according to this algorithm at that time.

Culture- Versus Risk-Based Screening

Early guidelines recommended the use of one of two approaches to identifying women who should receive intrapartum antibiotic prophylaxis: a risk-based approach or a culture-based screening approach. A large population-based study conducted during 1998-1999 demonstrated the superiority of culture-based screening over the risk-based approach to prevention of early-onset GBS disease. The study found that culture-based screening resulted in the identification of a greater proportion of women at risk for transmitting GBS to their newborns.

Risk-Based Method	Culture-Based Method
Presence of any of the following intrapartum risk factors: Delivery at <37 weeks' gestation Intrapartum temperature ≥100.4ºF (≥38.0ºC) Rupture of membranes for ≥18 hours	All pregnant women between 35 and 37 week's gestation undergo culture searching for: Vaginal GBS colonization Rectal GBS colonization

Specimen Collection and Processing for GBS Screening

Specimen Collection

Swabbing both the lower vagina and rectum (through the anal sphincter) increases the culture yield substantially compared with sampling the cervix or the vagina without also swabbing the rectum.

Although a small number of studies have examined the ability of perianal or vaginal-perianal cultures to detect GBS colonization, the available data on their performance compared with vaginal-rectal cultures are limited.

Studies have indicated that when women in the outpatient clinic setting collect their own vaginal-rectal screening specimens, with appropriate instruction, GBS yield is similar to when specimens are collected by a health-care provider.

The use of appropriate transport media can help sustain the viability of GBS in settings where immediate laboratory processing is not possible. GBS isolates can remain viable in transport media for several days at room temperature; however, the recovery of isolates declines during 1-4 days, particularly at high temperatures. Even when appropriate transport media are used, the sensitivity of culture is greatest when the specimen is stored at 4°C before culture and processed within 24 hours of collection.

Procedures for collecting clinical specimens for culture of group B Streptococcus (GBS) at 35--37 weeks' gestation

Swab the lower vagina (vaginal introitus), followed by the rectum (i.e., insert swab through the anal sphincter) using the same swab or two different swabs. Cultures should be collected in the outpatient setting by the health-care provider or, with appropriate instruction, by the patient herself. Cervical, perianal, perirectal or perineal specimens are not acceptable, and a speculum should not be used for culture collection. Place the swab(s) into a non-nutritive transport medium. Appropriate transport systems (e.g., Stuart's or Amies with or without charcoal) are commercially available. GBS isolates can remain viable in transport media for several days at room temperature; however the recovery of isolates declines over one to four days, especially at elevated temperatures, which can lead to false-negative results. When feasible, specimens should be refrigerated before processing. Specimen requisitions should indicate clearly that specimens are for group B streptococcal testing. Patients who state that they are allergic to penicillin should be evaluated for risk for anaphylaxis. If a woman is determined to be at high risk for anaphylaxis,* susceptibility testing for clindamycin and erythromycin should be ordered.

* Patients with a history of any of the following after receiving penicillin or a cephalosporin are considered to be at high risk for anaphylaxis: anaphylaxis, angioedema, respiratory distress, or urticaria.

Specimen Processing

Regardless of the test selected to identify GBS, use of an enrichment broth improves detection substantially. When direct agar plating is used instead of selective enrichment broth, as many as 50% of women who are GBS carriers have false-negative culture results. Examples of selective enrichment broths include Todd-Hewitt broth supplemented either with gentamicin (8 µg/ml) and nalidixic acid (15 µg/ml) [TransVag broth] or with colistin (10 µg/ml) and nalidixic acid (15 µg/ml) [Lim broth]. Although TransVag and Lim broth media are often available without blood, the addition of 5% sheep blood can increase the recovery of GBS . Selective enrichment broth also can contain chromogenic substrates that provide for a change in color in the setting of beta-hemolytic GBS. Such broths can facilitate the identification of beta-hemolytic GBS; however, non-hemolytic isolates will not be detected by these broths alone.

Following enrichment, the conventional means for identifying GBS is through isolation on subculture to blood agar plates and presumptive identification by the CAMP test or serologic identification using latex agglutination with group B streptococcal antisera. More recently, chromogenic agars that undergo color change in the presence of beta-hemolytic colonies of GBS have become available. As with pigmented enrichment broths, these chromogenic agars can facilitate detection of beta-hemolytic GBS, but the majority will not detect non-hemolytic strains. In addition more rapid techniques for identifying GBS directly from enrichment broth, or after subculture have been developed, including DNA probes and nucleic acid amplification tests (NAAT) such as polymerase chain reaction.

Despite the availability of NAAT for GBS, utility of such assays in the intrapartum setting remains limited. Although a highly sensitive and specific test with rapid turnaround time could be used to assess intrapartum GBS colonization and therefore obviate the need for antenatal screening, data on currently available assays do not support their use in replacement of antenatal culture or risk-based assessment of women with unknown GBS status on admission for labor. The additional time required for enrichment of samples makes it not feasible for intrapartum testing, and the sensitivity of assays in the absence of enrichment is not adequate in comparison to culture. In addition, concerns remain regarding real-world turnaround time, test complexity, availability of testing at all times, staffing requirements, and costs. In settings that can perform NAAT, such tests might prove useful for the limited circumstance of a woman at term with unknown colonization status and no other risk factors. Even optimal NAAT would have drawbacks in the intrapartum setting, including a delay in administration of antibiotics while waiting for the result, and no antimicrobial susceptibility testing for penicillin-allergic women. Other rapid tests in addition to NAAT have been developed to detect GBS rapidly from non-enriched samples, including optical immunoassays and enzyme immunoassays; however, none is sufficiently sensitive when used on a direct specimen to detect GBS colonization reliably in the intrapartum setting.

Antimicrobial Susceptibility Testing

Antimicrobial susceptibility testing of GBS isolates is crucial for appropriate antibiotic prophylaxis selection for penicillin-allergic women who are at high risk for anaphylaxis because resistance to clindamycin, the most common agent used in this population, is increasing among GBS isolates.

In addition, appropriate methodologies for susceptibility testing are important because inducible clindamycin resistance can occur in some strains that appear susceptible in broth susceptibility tests. D-zone testing using the double-disk diffusion method has been used to identify isolates that are erythromycin-resistant and clindamycin-susceptible, yet have inducible resistance to clindamycin. Isolates that are D-zone positive are considered to have inducible clindamycin resistance and are presumed to be resistant although the clinical significance of this resistance is not clear.

Identification of Group B Streptococcus (GBS) Bacteriuria in Pregnant Women

Routine screening for asymptomatic bacteriuria is recommended in pregnant women, and laboratories should screen urine culture specimens for the presence of GBS in concentrations of 104 colony-forming units (cfu)/ml or greater.

Laboratories should identify GBS when present at ≥104 cfu/ml in pure culture or mixed with a second microorganism.

References

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