Objective
We sought to evaluate whether group B streptococcus (GBS) detection is altered by the digital cervical examination.
Study Design
A total of 302 women undergoing the clinical GBS culture had a digital cervical examination and a repeated GBS culture. Statistical comparison of pre–post culture results were performed with kappa and McNemar tests.
Results
The clinical prevalence of GBS was 19.5%. Discordant results were seen in 30/302 (9.9%) paired cultures (κ = 0.68; 95% confidence interval, 0.568–0.783). An initially negative GBS culture result was positive on repeated testing in 13/243 (5.3%) pairs. Initially positive cultures were negative on repeated testing in 17/59 (28.8%) pairs. Patients with discordant results had similar characteristics as the remainder of the study group. Given the observed proportion of discordant results (9.9%), the study had 80% power to detect a 5% difference between discordant pairs.
Conclusion
Paired GBS cultures showed a good level of agreement. The 28.8% rate of positive cultures becoming negative is clinically concerning and warrants further study.
Group B streptococcus (GBS) colonizes up to 30% of pregnant women and remains a leading cause of neonatal infection. Neonatal GBS acquired at delivery can lead to sepsis, pneumonia, meningitis, and death. Historically, the decision by providers to give intrapartum prophylaxis was determined either by risk factors, or routine screening cultures performed at 35-37 weeks. The Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists have revised their guidelines, and now recommend universal screening of all pregnant women. Women who screen positive by rectovaginal cultures receive intrapartum antibiotic prophylaxis, typically in the form of penicillin.
Despite dramatic reductions in disease incidence, up to 1600 cases and 80 deaths are expected to occur annually. The adoption of universal screening is anticipated to further reduce the disease burden, but remains imperfect. The reported efficacy of the culture-based screening method is approximately 90%. Reasons for screening and treatment failures are not completely understood, but include antibiotic resistance and failure to administer or complete the antibiotic regimen.
The University of Oklahoma Health Sciences Center serves as the regional referral location for the state of Oklahoma, and approximately 600 maternal transports are received annually. It is not uncommon in our setting for a woman to have been evaluated with a digital cervical examination outside our facility, and require a GBS culture upon arrival. At present there are no clear guidelines regarding this practice. This contrasts with the fetal fibronectin test. Testing is not recommended within 24 hours of digital cervical examination because both false-positive and false-negative results can occur.
We hypothesized that one factor affecting the efficiency of universal screening may be examinations of the lower genital tract. Our aim was to determine whether there was a difference in the detection of GBS when paired cultures were performed before and immediately after digital cervical examination.
Materials and Methods
The study used a pre–post design between June 2007 and May 2008. Appropriate institutional review board consent was obtained to recruit patients and perform cultures in 2 clinics (high-risk obstetric clinic and women’s clinic) at the University of Oklahoma Health Sciences Center.
Women who were preparing to undergo routine third-trimester (35-37 weeks) GBS screening were recruited sequentially to participate. Participants (n = 367) had their initial GBS culture done according to the CDC guidelines. Briefly, without using a speculum, a swab of the vaginal introitus, followed by a swab of the rectum just inside the anal sphincter, was performed. The swabs were placed into a transport medium (BBL Culture Swab Plus; Becton Dickinson, Sparks, MD), with 5 mL of Amies agar (Becton Dickinson). The appropriate clinical forms were completed, including any penicillin allergies, and requests for sensitivity testing. This first clinical specimen was made available for clinical care.
Study participants then had an examination of their cervix followed by repeated GBS testing. The digital cervical examination was performed with the contents of 1 package of sterile water-soluble lubricant (E-Z Lubricating Jelly; Chester Labs, Cincinnati, OH). The clinical assessment of the cervix was recorded in the medical chart. Next, a repeated GBS culture was performed in an identical manner by the same practitioner. Microbiology personnel were blinded to the study specimen by labeling the second culture with a number. Study cultures were collected in batch fashion each weekday and delivered to the microbiology laboratory separately from the clinical specimen by our study nurse. These results were not made available for clinical care.
At specimen collection, recorded demographic data included the patient’s age, race, gravidity, parity, the presence of obstetric or medical complications, smoking, and type and interval of recent antibiotic exposure. Obstetric complications were defined as: any class of diabetes, hypertensive disorder of pregnancy, preterm labor, fetal abnormality, and any sexually transmitted diseases during pregnancy ( Chlamydia , gonorrhea, and/or syphilis). Recorded maternal medical complications were: chronic hypertension, seizure disorder, congenital or acquired cardiac disease, asthma, and/or thyroid disease. We also noted the collection and processing date and time of clinical and study cultures.
Per CDC recommendations, any clinical or study culture was excluded if not processed within 96 hours of collection. We also excluded patients with preterm premature ruptured membranes, and those who had sexual intercourse or a vaginal examination within the preceding 24 hours.
Our null hypothesis was that digital cervical examination would have no impact on the detection of GBS. Based on historical controls, we assumed a 30% GBS carrier rate in our pregnant population and computed that a sample size of 293 paired samples would provide >80% power to detect a 30% difference in the incidence for a 2-sided test of significance at a critical level of .05. McNemar χ 2 tested the proportion of paired samples that had dissimilar results. A P value of < .05 was considered statistically significant. A kappa statistic was calculated to estimate percent agreement beyond chance between preexamination and postexamination test results. The kappa statistic was interpreted according to the standards reported by Altman : <0.20 poor agreement, 0.21-0.40 fair agreement, 0.41-0.60 moderate agreement, 0.61-0.80 good agreement, and 0.81-1.00 very good agreement.
Results
A total of 367 patients were recruited for this study. After the first 65 patients were recruited, a protocol violation was discovered. At the resident high-risk obstetric clinic, an unknown number of patients had their paired cultures collected by different personnel. Efforts to identify the patient samples collected in error were futile, and the decision was made to temporarily halt the trial. Research and clinic personnel were retrained, the institutional review board was notified, and the study was restarted. None of the initial 65 patients appear in the final analysis.
A total of 302 patients were subsequently recruited and included for study. Mean maternal age was 27.2 ± 6.0 years. The pretest culture was positive in 59 of 302 patients for a clinical GBS prevalence of 19.5%. Further characteristics of the study group are featured in Table 1 .
| Characteristic | n (%) |
|---|---|
| Maternal age, y | |
| <25 | 112 (37.1) |
| ≥25 | 190 (62.9) |
| Race/ethnicity | |
| African American | 60 (19.9) |
| Other | 242 (80.1) |
| Gestational age, wk.d | |
| 25.6-34.6 | 26 (8.6) |
| 35.0-36.6 | 232 (76.8) |
| 37.0-39.3 | 43 (14.2) |
| Missing | 1 (0.3) |
| Parity | |
| 0 | 78 (25.8) |
| ≥1 | 224 (74.2) |
| Gravidity | |
| 1 | 59 (19.5) |
| ≥2 | 243 (80.5) |
| Smoker | |
| No | 249 (82.5) |
| Yes | 52 (17.2) |
| Missing | 1 (0.3) |
| Obstetric complications and preexisting conditions a | |
| None | 148 (49.0) |
| ≥1 | 154 (51.0) |
| Last antibiotic use | |
| Within last 7 d | 13 (4.3) |
| 8-60 d | 32 (10.6) |
| 61-365 d | 17 (5.6) |
| Timing unknown | 2 (0.7) |
| None in past year | 238 (78.8) |
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