By Louise Lazear
Despite great strides to reduce perinatal infection in the 1990s, Group B streptococcus (GBS) remains a major cause of serious neonatal infection, leading to sepsis,meningitis, and in an estimated five to 20 percent of infected newborns, death. In the 1970s, GBS was considered the leading cause of newborn morbidity and mortality, with reported case fatality rates reaching 50 percent. In the 1980s, researchers discovered that antibiotic therapy for women at risk of transmitting GBS to children had the ability to prevent early onset of the disease. As a result of this research, in 1996 the CDC published guidelines for predelivery prophylaxis of women utilizing either a risk-based approach or a screening approach employing standardized culture-based techniques.
Data comparing the effectiveness of these two approaches prompted the CDC to revisit the guidelines, and new recommendations for diagnosis, treatment and management of GBS were published in August 2002. While many of the previous recommendations remain in place, the new guidelines will have a significant impact on microbiology laboratories, especially for those in women’s healthcare and labor and delivery settings.
“Based on new evidence now available, we decided to recommend universal screening of all pregnant women late in pregnancy, as opposed to the previous guidelines which allowed providers to choose between culture-based and risk-based strategies,” said Stephanie Schrag, PhD, epidemiologist at the CDC and primary author of the new recommendations. “There are a number of other changes that impact clinical management. The big change for labs, however, is that the volume of specimens will greatly increase because all providers should be using this screening-based approach. The other impact from the laboratory’s perspective is that we now also recommend susceptibility testing of GBS isolates in women at risk for penicillin anaphylaxis,” she added.
According to Schrag, the CDC guidelines for laboratory analysis of GBS identification remain relatively unchanged. Recommendations include initial incubation in either Lim broth or selective broth medium, then subculture onto sheep blood agar, inspection for organisms suggestive of GBS, and confirmation of GBS using streptococcus grouping latex agglutitnations tests or genetic probes, or the cyclic adenosine monophosphate (CAMP) test reaction for presumptive identification.
In order to help reduce time-consuming multiple inoculation and incubation procedures, Northeast Laboratory Services has developed new media for the isolation and direct identification of beta-hemolytic Streptococcus agalactiae. Available in both broth and agar, the medium produces positive results indicated by the presence of an orange color change of the medium. Earliest results may be read at four hours with final results available at 16 to 24 hours. The media can also be used to transport swabs to the laboratory, and has the potential to be used in the physician office, clinic, or delivery room. According to NEL, use of their medium results in higher than 96 percent sensitivity and specificity for beta-hemolytic GBS based on preliminary studies.
“For maximum sensitivity of GBS detection, the CDC recommends the use of a selective broth containing antibiotics, such as LIM broth,” said Lauraly LeBrasseur, MT, AMT and quality assurance manager at NEL. “The NEL-GBS media meets these recommendations, as it is selective and contains antibiotics. What is special about the media is that one can see a positive result with overnight incubation, which saves 48 to 72 hours of incubation and technical time, an obvious benefit to the patient, especially if it is a neonate or newborn in danger of GBS infection. Because the media is highly specific to beta-hemolytic strains only, we recommend subculture of all negative specimens using traditional GBS identifying media.” Subcultures may be taken directly from either broth or agar media if further testing is required, such as antibiotic sensitivity testing.
While similar to Granada broth, according to LeBrasseur the color change is a result of the presence of a beta-carotene pigment that reacts with proprietary constituents in the NEL-GBS media. Because the broth has a milky appearance, inoculation of bloody swabs into the broth may mask the color change, and the agar medium, which requires anaerobic incubation is recommended for this situation. The company suggests the use of coverslips over the area of the initial inoculum to create anaerobic conditions when incubators are not available. The broth may also be used as a transport medium. “The broth is stable for two to three days, which means that if the sample was taken on a Friday night and it doesn’t arrive in the lab until Monday, the media will still support the organism,” she added.
According to LeBrasseur, the product has been used in the European community for several years. In 2001 researchers in the Czech Republic compared the sensitivity of GBS detection in vaginal and rectal samples obtained from women in labor using three different media: Todd-Hewitt selective broth, GBS agar and GBS broth. Positive results from both vaginal and rectal samples from the respective media were evaluated. The sensitivity of the standard method using Todd-Hewitt broth for the detection of vaginal and rectal colonization of GBS was 97 and 90.9 percent respectively. The corresponding values for GBS agar were 93.9 and 92.2 percent, and for GBS broth were 89.4 and 87. When combined results from both the rectal and vaginal samples were considered, the sensitivities of the three media were almost equal: 97.8, 97.8 and 96.6 percent respectively. These results led the researchers to conclude that GBS media can be used to replace selective Todd-Hewitt broth, the advantages seen as a reduction in costs as well as a decreased time to detection.
In the US, three studies are currently underway to evaluate the sensitivity and specificity of the NEL-GBS medium, as well as an evaluation for the need to perform anaerobic incubation. The Clinical Microbiology Laboratory at Women and Infants Hospital in Providence, Rhode Island is evaluating the use of NEL-GBS Medium as a replacement for their traditional method of using THB with Gentamicin and Naladixic Acid as a direct transport and culture medium. “We are an obstetrics and gynecology hospital and primary care facility for women, and we handle 9,000 deliveries per year. We have been following the ACOG GBS screening recommendations for years and we process 700 GBS screens per month for our hospital and outreach clients,” said Judith Struminsky, co-supervisor of the Women and Infants’ clinical microbiology laboratory, along with Patricia Lauro. “We saw a product announcement about the NEL-GBS medium, and felt that if it performed as well as our traditional technique, it would be the perfect replacement for our current procedure.”
Struminsky and Lauro developed a research protocol comparing NEL-GBS media to their current method, which for many samples involves a four-day process of subcultures, incubations and presumptive CAMP testing. With IRB approval, the hospital approached physicians at their outreach sites to perform parallel sample collections during routine GBS screening. Dual-tipped swabs are used to obtain a vaginal/rectal specimen. After collection, the swabs are placed into two tubes, one containing traditional media and one NEL-GBS medium. After incubation for 24 hours, NEL-GBS medium initially positive results are recorded, and both tube sets undergo subculture using identical techniques. After subculture, samples are evaluated for the presence of GBS colonization. “We have completed 250 of our planned 300 parallel tests, and at this point, our results are extremely promising. We are very excited about the progress of the study so far,” said Struminsky.
Because 25 percent of GBS screens in their laboratory are positive, Struminksy feels that the new media, if implemented, will have a significant impact on both laboratory technical time, time to result, and costs. “The next step in our research is to perform an analysis of the cost and time savings realized with the use of the new media,” she said.
While other approaches to GBS testing evolve, including the development of rapid analysis techniques at the time of delivery, microbiology laboratories will feel the impact of the new CDC recommendations. However, because of the research being performed at Women and Infants and other facilities, it is likely that other laboratories will begin to investigate the use of GBS media to help alleviate additional workload, increase time to result, and defray costs.
Louise Lazear is a freelance writer based in Charlotte, N.C.