Group B streptococci (GBS) is an established cause of neonatal morbidity and mortality. Approximately 10–30% of pregnant women are colonized with GBS.1,2 In the 1970s, the vertical transmission rate was 1.8 per 1,000 live births and the case fatality rate of neonatal infections was as high as 50%.3 During the 1980s, it was determined that intrapartum antibiotics could reduce the risk of vertical transmission.4 The Centers for Disease Control and Prevention (CDC) released guidelines in 1996 that recommended using either routine screening or a risk-based protocol to determine the need for intrapartum antibiotic prophylaxis. A subsequent study in 2000 demonstrated a 65% decline in the incidence of early-onset neonatal GBS infections.5 Despite this reduction, early-onset GBS disease remained a leading cause of infectious neonatal morbidity and mortality.6,7
In 2002, the CDC released new guidelines recommending universal prenatal screening. The procedure for collecting clinical specimens for GBS cultures between 35 and 37 weeks of gestation was outlined by the CDC and the American College of Obstetricians and Gynecologists (the College). The protocol stated that a swab of “the lower vagina (vaginal introitus) followed by the rectum (ie, insert swab through the anal sphincter)” is to be used for detection of GBS.1,2 The protocol was unchanged in the updated 2011 College guidelines.8 This recommended method of collection is based on a 1977 study that suggested that the gastrointestinal tract was the primary site of GBS colonization, as 17.9% of rectal cultures from gravid women were positive compared with 10.2% of vaginal cultures.9 After adoption of the 2002 guidelines, the incidence of early-onset GBS disease decreased to 0.34 per 1,000 live births with a case fatality rate of 6.8%.10
Since implementation of the protocol for universal screening, three studies have assessed various collection methods for determination of GBS colonization.11–13 All three studies concluded that there is no statistical superiority in vaginal–rectal cultures when compared with alternative methods. However, none of the previous studies used a single swab for vaginal–rectal and another for vaginal–perianal cultures nor looked at patient reported pain and discomfort with the different collection methods. The purpose of this prospective cohort study was to estimate whether the agreement between test results obtained with vaginal–perianal cultures and vaginal–rectal cultures is sufficient to employ vaginal–perianal cultures for detection of GBS colonization in pregnant women. The study also looked at patient-reported pain and discomfort associated with the two collection methods.
MATERIALS AND METHODS
This prospective cohort study was carried out at Riverside Methodist Hospital's Obstetrics and Gynecology Community Care Center located in Columbus, Ohio. After receiving approval from the hospital's Institutional Review Board, data were collected from pregnant women between October 2009 and April 2010. Inclusion criteria were pregnant women aged 18 or older who were to undergo the routine GBS vaginal–rectal swab who spoke English, Somali, or Spanish. The schedule was checked weekly for patients arriving for their 35- to 37-week prenatal care appointment. Patient records were assessed for eligibility and all consecutive eligible patients were invited to participate in the study. Eighteen obstetric-gynecologic residents and one nurse practitioner collected cultures after obtaining written informed consent. All participating practitioners were orientated to the study and collection techniques were standardized.
The vaginal–perianal culture was collected first to avoid potential contamination of the perianal region by the rectal swab. The vaginal–perianal swab was collected by inserting the swab into the lower vagina and then, using the same swab, sampling the perianal surface without penetration through the anal sphincter into the rectum. This specimen was then placed into transport medium and labeled with patient identification and “specimen #1 V-P.” Immediately after collection of the vaginal–perianal culture, the standard vaginal–rectal culture was collected by following the guidelines of the College and the CDC: the swab was entered into the “lower vagina (vaginal introitus) followed by the rectum (ie, insert swab through the anal sphincter).”1,2 This specimen was then placed into transport medium and labeled with patient identification and “specimen #2 V-R.” Specimens were stored at room temperature (BD CultureSwab) before transport to the laboratory on average about 4 hours later and always within 24 hours of collection.
Both cultures were sent to the Riverside Methodist Hospital Laboratory for detection of GBS. The specimens were processed separately under the aforementioned labels in accordance with CDC guidelines; using the swab to inoculate Lim broth followed by immediate incubation at 35°C (5% CO2 incubator), then testing the Lim broth for the presence of GBS using the GEN-Probe Accuprobe GBS culture identification test 18–24 hours after inoculation. If either culture returned positive for GBS, the patient was considered to be a carrier and intrapartum prophylaxis was provided in accordance with the guidelines in effect at the time of data collection.
After the two specimens were collected, enrolled patients completed a short written questionnaire in which they were asked to rate their pain on a 0–10 scale for both collection methods (described as “first test” and “second test,” with “0” meaning “no pain” and “10” meaning “extremely severe pain”) and then compare the discomfort from the first test to that of the second test (choices were less discomfort with first test, same amount of discomfort with both tests, more discomfort with first test).
The primary outcome of the study was the proportion of patients whose vaginal–perianal and vaginal–rectal culture results agreed with one another. The agreement rate between the two specimen-collection methods was calculated along with its 95% upper one-sided (confidence interval [CI]) based on the adjusted Wald method. The objective was to determine whether the agreement rate between the vaginal–perianal and vaginal–rectal methods was 90.0% or higher. The binomial test for superiority (with P0=.9) was done to assess whether the observed point estimate for the agreement rate was consistent with more than 90%. Under the assumption that the point estimate for the agreement rate in this study would be at least 93%, the calculated sample size was 196.
In addition, the McNemar test was used to compare the two collection methods to evaluate the likelihood that one collection method was more likely to produce a positive result than the other. Using the vaginal–rectal method as the gold standard, the sensitivity, specificity, and positive and negative predictive values were also calculated for the vaginal–perianal method, along with their 95% CIs using the adjusted Wald method. The comparison of patient reported pain levels between the two collection methods was performed using the Wilcoxon signed ranks test.
There were 200 patients who consented to participate in the study. Seven patients were later excluded after it was determined that their gestational age fell outside the range for inclusion in the study, leaving 193 patients in the analysis. Their demographic and pregnancy characteristics appear in Table 1. Women were on average in their mid-20s and represented various ethnic backgrounds. Results of the GBS tests, which were reported from the laboratory as “positive” or “negative,” separated by collection method can be seen in Table 2. The overall agreement rate was 96.4% (186/193) with a 95% confidence boundary on the agreement rate of 93.4%. The P value for the binomial test was .003, indicating that the agreement rate was greater than 90%. For reference purposes, the two-sided 95% CI for the agreement rate is 92.6% to 98.4%. McNemar test returned a P value of 0.257, indicating that neither collection method was more likely to report a positive GBS test result than the other.
Table 3 provides data on the screening parameters of the vaginal–perianal method, using the vaginal–rectal method as the gold standard. Sensitivity was 91.1%, while the performance of the other parameters was at least 96%. If the recommended vaginal–rectal method is considered the gold standard for detecting GBS, the vaginal–perianal method missed five cases for a false negative rate of 8.9%; in addition, the lower confidence bound for sensitivity was 83.6%.
Patient reported pain with each type of collection method, as well as discomfort between the two methods is presented in Table 4. Pain level was on average 2.2 points higher with the vaginal–rectal method (P<.001). Twenty-five percent of patients rated their pain as 6 or higher with the vaginal–rectal method, whereas only 3% rated their pain as 6 or higher with the vaginal–perianal method. Based on the numeric pain levels, 10 (5.2%) patients rated their pain higher with the vaginal–perianal method, and 127 (65.8%) patients rated their pain as worse with the vaginal–rectal method. Ninety-eight (50.8%) patients had no pain (pain level=0) with the vaginal–perianal method, compared with 36 (18.7%) for the vaginal–rectal method. When asked to compare the discomfort levels of the two specimen-collection methods, more than two-thirds of the patients stated that the vaginal–perianal method had less discomfort than the vaginal–rectal method.
The purpose of this prospective cohort study was to determine whether vaginal–perianal cultures could be an alternative to the recommended vaginal–rectal cultures for the detection of GBS in pregnant women. The study also looked at patient reported pain and discomfort of the two collection methods. The results of this study suggest that vaginal–perianal cultures can be an acceptable alternative for vaginal–rectal cultures as indicated by an agreement rate of 96.4% and a confidence bound greater than 90%. The overall GBS detection rate was 29% (56/193), which is similar to that for other studies,1,2,11–13 suggesting that our results are relevant to other populations. In general, the results are consistent with other studies showing no superiority of anorectal cultures. Future studies could monitor early-onset GBS disease in neonates after switching to vaginal–perianal collection methods because neonatal health status was not followed in this study.
Previous studies have evaluated various collection methods. Orafu et al11 performed a prospective cohort study of 136 participants between 33 and 40 weeks of gestation with collection of three samples: perianal, vaginoperianal, and anorectal. The results of this study showed no statistical difference in detection rate among the three samples collected. The vaginal–perianal detection rate was 26.5%, anorectal was 27.2%, and perianal was 28.7%. Participants were asked about pain in relation to the anorectal collection method (none, mild, moderate, severe).
Jamie et al12 performed a similar study in which vaginal, perianal, and rectal cultures were collected from 200 participants between 28 and 42 weeks of gestation. The results of these cultures were combined to compare vaginal–perianal and vaginal–rectal cultures. No statistical difference was found in the detection rate of GBS: 34% for the vaginal–perianal and 33.5% for the vaginal–rectal cultures.
In 2007, Kovavisarach et al13 collected vaginal and anorectal cultures from 320 individuals between 28 and 42 weeks of gestation and extrapolated vaginal–anorectal results by combining the results of the two cultures. No difference was noted between vaginal and anorectal cultures, contradicting the 1977 study on which the CDC and College collection method for GBS cultures is based. There was statistical improvement in GBS detection using vaginal–anorectal compared with the use of single sites; vaginal swab detected 13%, anorectal 10%, and the two combined 18%.
Despite the aforementioned studies, no change has been made to the recommended method of collection for GBS cultures. Therefore, this study was designed to compare the recommended method with a potentially more patient-friendly alternative method. In contrast to the previous studies, this study uses the single-swab specimen-collection method for GBS screening commonly used in practice to specifically compare vaginal–perianal to vaginal–rectal detection rates. The agreement was high between the recommended vaginal–rectal and the experimental vaginal–perianal collection methods with multiple caregivers involved in specimen collection. This study only includes patients between 35 and 37 weeks of gestation in accordance with College and CDC screening guidelines. This study also had patients report pain and discomfort for each culture method and found that the vaginal–perianal collection method involved less pain and discomfort. The degree of the pain reduction is considered clinically relevant as identified by previous pain studies.14,15
This study adds to the current body of evidence that suggests that vaginal–perianal cultures may be a reasonable, patient-preferred alternative for the collection of recommended cultures for detection of GBS during pregnancy.
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© 2011 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
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