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Vaginal–Perianal Compared With Vaginal–Rectal Cultures for Identification of Group B Streptococci

Jamie, Whitney E. MD; Edwards, Rodney K. MD, MS; Duff, Patrick MD

doi: 10.1097/01.AOG.0000144120.20312.ed
Original Research

OBJECTIVE: To estimate whether the rates of recovery of group B streptococci from combined vaginal and perianal cultures and combined vaginal and rectal cultures are equivalent.

METHODS: We performed a prospective cohort study of vaginal–perianal versus vaginal–rectal culture for group B streptococci. Two hundred pregnant women in the third trimester were enrolled. Three specimens were collected from each patient in the following order: lower third of the vagina, perianal skin, and rectum. Each specimen was cultured serially in selective broth media, then on sheep's blood agar. Suspicious colonies were confirmed by latex agglutination. Culture positivity rates from the combined sites of vagina and perianal skin were compared with vagina and rectum. Laboratory personnel were blinded to the collection site of each individual swab.

RESULTS: Of the 200 subjects, 71 (36%) had a positive culture from at least 1 site. Vaginal culture was positive in 55 patients (28%), compared with 48 patients (24%) with positive perianal cultures and 50 patients (25%) with positive rectal cultures. Results of combined vaginal and perianal cultures were positive in 68 patients (34%); results of combined vaginal and rectal cultures were positive in 67 patients (34%) (P = 1.0).

CONCLUSION: The group B streptococci detection rate from vaginal–perianal specimens is not significantly different from the detection rate from vaginal–rectal specimens. Therefore, pregnant women do not need to be subjected to the discomfort of collection of a rectal specimen.


Vaginal–perianal and vaginal–rectal specimens are equivalent for detection of group B streptococcal colonization in pregnant women.

From the Department of Obstetrics and Gynecology, Division of Maternal–Fetal Medicine, University of Florida College of Medicine, Gainesville, Florida.

Received May 24, 2004. Received in revised form August 16, 2004. Accepted August 18, 2004.

This study was supported by departmental funds.

Reprints are not available. Address correspondence to: Whitney E. Jamie, MD, University of Florida, Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, Box 100294, Gainesville, FL 32610; e-mail:

Streptococcus agalactiae, or group B streptococci, was identified as a major cause of neonatal sepsis in the 1970s.1 Approximately 2 per 1,000 live births are affected, with a case-fatality rate of 5–10% for cases of early-onset neonatal sepsis.2 Previous trials have shown a decreased risk of neonatal group B streptococcal sepsis when mothers received intrapartum antibiotic prophylaxis.3–6 Recommendations for intrapartum antibiotic prophylaxis against early-onset neonatal group B streptococcal disease were issued by the American College of Obstetricians and Gynecologists, American Academy of Pediatrics, and the Centers for Disease Control and Prevention (CDC) in 1996.7 The CDC released a revised version of the guidelines in August 2002.8 The revised guidelines recommended screening all pregnant women for group B streptococcal colonization at 35–37 weeks gestation and administering intrapartum antibiotic prophylaxis to all colonized women. The guidelines also indicate that women with unknown culture results should receive prophylaxis if they deliver before 37 weeks gestation, have an intrapartum fever, have ruptured membranes for longer than 18 hours, or have delivered a prior infant with early-onset infection with group B streptococci.8

The method of specimen collection suggested by the CDC guidelines involves swabbing both the lower third of the vagina and rectum. This method is based on a study from 1977, which found that rectal cultures were positive more often than vaginal cultures (17.9% versus 10.2%).9 A more recent study, published in 2002, compared cultures taken from the rectum with those taken from the perianal area and from the perianal area and vagina combined. These investigators found that there was no significant difference in recovery of group B streptococci and that almost three fourths of women reported at least mild pain associated with obtaining the rectal culture.10 However, this study was small and did not include vaginal culture combined with rectal culture, as recommended in the CDC guidelines.

Our objective in the current study was to evaluate whether combined vaginal and perianal culture is equivalent to combined vaginal and rectal culture for detection of group B streptococcal colonization of pregnant women. If the information obtained from perianal and rectal cultures is indeed the same, then patients can be spared an uncomfortable screening test without compromising the detection of mother–infant pairs at risk of vertical transmission of group B streptococcal infection.

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We performed a prospective cohort study to estimate the rate of recovery of group B streptococci from cultures taken from the vagina, perianal skin, and rectum. Women were eligible for the study if they were receiving prenatal care in the University of Florida clinic system. Women in the third trimester of pregnancy during the study period, from July 2003 to October 2003, were invited to participate. The only exclusion criteria were declination of participation and the presence of ruptured membranes. Approval for this study was obtained from the University of Florida Health Center Institutional Review Board.

After written informed consent was obtained from the patient, 3 swabs were used to collect specimens from the lower third of the vagina, perianal skin, and rectum. The swabs were collected in this order to avoid potential contamination of the perianal site by the rectal swab. The samples were obtained before vaginal examination with bacteriostatic gel. These 3 swabs were separate from the routine swab used for clinical management. If a patient had a clinical specimen collected at the same visit, it was obtained after the research specimens and sent to the hospital's clinical laboratory. The physicians at our institution currently obtain either vaginal–perianal or vaginal–rectal cultures at their own discretion. Demographic data including age, race, marital status, parity, insurance status, and gestational age also were collected at the time of sample acquisition.

The swabs were maintained at room temperature in modified Stuart's transport media. Within 24 hours, they were taken to the Infectious Diseases Research Laboratory of the Division of Maternal–Fetal Medicine, inoculated into TransVag broth (Remel, Doraville, GA), and incubated at 35°C for 18–24 hours. After incubation, a small amount of the broth was streaked onto 5% sheep's blood agar plates, and the plates were incubated at 35°C for another 18–24 hours. The plates then were read, and suspicious colonies were confirmed as group B streptococci with latex agglutination testing. This method of isolation and identification of group B streptococci is consistent with the CDC guidelines.8 The swabs always were collected in the same order; however, laboratory personnel were blinded to the site of collection for each swab when reading the plates. A single investigator (W.E.J.) performed all the cultures.

Descriptive statistics of demographic data were generated using means and proportions. The primary outcome variable in this study was the proportion of cultures from each site that was positive for group B streptococci. The required sample size was calculated using StatXact 5 for Windows (Cytel Software Corp., Cambridge, MA). The exact sample size for the McNemar test for paired binomial proportions was used. Assuming 20% of subjects would have a positive result by both methods (vaginal–perianal and vaginal–rectal) and 70% of subjects would have a negative result by both methods, 184 subjects would be required to detect a 6% difference between the subjects with nonconcordant results (Table 1). This calculation used an α of .05 and 1 − β of .80. Therefore, 200 subjects were enrolled.

Table 1

Table 1

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Two hundred women were enrolled in the study. Their mean age was 25.0 ± 5.5 years (range 18–41 years). Fifty-eight percent of the women were white, 29% were African American, 8% were Hispanic, and 4.5% were of other ethnicities. Fifty-seven percent were parous, and 44% were married. The majority of the patients (79%) were funded by Medicaid. The mean gestational age at the time of enrollment was 35.4 ± 3.1 weeks (range 28–42 weeks).

Of the 200 women, 71 (35.5%; 95% confidence interval [CI] 30–43) had a positive culture result from at least 1 site. Vaginal culture results were positive in 55 patients (27.5%; 95% CI 22–35), compared with 48 patients (24%; 95% CI 19–30) with positive perianal culture results and 50 patients (25%; 95% CI 20–31) with positive rectal culture results. Combined vaginal and perianal culture results were positive in 68 patients (34%; 95% CI 28–41), and combined vaginal and rectal culture results were positive in 67 patients (33.5%; 95% CI 28–41). There was no significant difference between the combined groups in the detection rate of group B streptococci (P = 1.0) (Table 2). Vaginal culture alone and perianal culture alone were compared with combined vaginal–perianal culture, and both individual cultures were found to have significantly lower detection rates than the combined culture (Tables 3 and 4). Rectal culture alone also detected significantly fewer colonized women than did combined vaginal–perianal culture (25% versus 34%, P = .0005).

Table 2

Table 2

Table 3

Table 3

Table 4

Table 4

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Although the incidence of early-onset neonatal group B streptococcal infection has been reduced significantly by routine screening of patients and use of intrapartum antibiotic prophylaxis, group B streptococcal infection remains an important cause of neonatal morbidity and mortality. Strict adherence to a culture-based screening strategy is important to identify as many carriers of group B streptococci as possible. Several studies have evaluated which culture sites yield the highest rate of group B streptococci. MacDonald et al11 reported that specimens obtained from the lower vagina were more often positive than those from the cervix or upper vagina, and Rosa et al12 have shown that combined vaginal and perianal swabs are positive more often than vaginal swabs alone. The CDC guidelines from 2002 recommend culturing both the lower vagina and the rectum, based on the increased recovery when 2 sites are sampled. This method was chosen based on a small study from 1977 in which rectal cultures were positive in a higher percentage of women than were vaginal cultures, suggesting that the gastrointestinal tract may be the primary site of colonization and vaginal colonization may be the result of contamination.9

Our results are consistent with those of Orafu and colleagues,10 who concluded that perianal cultures were equivalent to anorectal cultures, and who showed that most patients reported discomfort due to collection of a rectal swab. However, those authors did not compare combined vaginal and perianal with combined vaginal and rectal cultures, which is the current method recommended by the CDC. Our hypothesis was that vaginal–perianal culture would be equivalent to vaginal–rectal culture. We have shown this to be this case in our patient population. The finding of equivalence between perianal and rectal specimens in more than 1 population is reassuring, and we believe that patients no longer need to be asked to comply with this uncomfortable screening test.

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© 2004 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.