select female candidates to iap: the culture-based screening approach and the risk-based approach. the culture-based screening approach identifies candidates using lower vaginal and rectal cultures obtained between 35 and 37 weeks of gestation (or 36-37), and iap is administered to all gbs colonized women. the risk-based strategy identifies candidates to receive iap by the aforementioned risk factors known to increase the probability of gbs-eod without considering if the mother is or is not a gbs carrier. iap is also recommended for women with intrapartum risk factors if their gbs carrier status is not known at the time of delivery, and for women with gbs bacteriuria (in any colony count) during their pregnancy, and for women who have had an infant with gbs-eod previously. the risk-based approach is, in general, less effective than the culture-based approach, because in most cases, gbs-eod develops among newborns who have been born to mothers without risk factors. iap is not required for women undergoing planned caesarean section in the absence of labour and with intact membranes, irrespective of the known gbs carriage status. acog recommendations state, “women with a positive prenatal gbs culture result who undergo a cesarean birth before the onset of labor and with intact membranes do not require gbs antibiotic prophylaxis”. this recommendation is based on the fact that infants delivered via pre-labor cesarean section have less exposure to gbs in the vagina and have lower rates of gbs infection. hence, despite the recommended universal recto-vaginal screening of all pregnant women at 36 0/7 to 37 6/7 weeks, women who undergo pre-labor cesarean sections do not need iap, regardless of the screening test results. routine screening of pregnant women is performed in most developed countries such as the united states, france, spain, belgium, canada, and australia, and data have shown falling incidences of gbs-eod following the introduction of screening-based measures to prevent gbs-eod. the risk-based strategy is advocated, among other counties, in the united kingdom, the netherlands, new zealand, and argentina. the issue of cost-effectiveness of both strategies for identifying candidates for iap is less clear-cut, and some studies have indicated that testing low risk women, plus iap administered to high-risk women, and to those found to carry gbs is more cost-effective than the current uk practice. other evaluations have also found the culture-based approach to be more cost-effective than the risk-based approach for the prevention of gbs-eod. testing pregnant women to detect gbs carriers and giving iap to those carrying gbs and to high-risk women has also been proposed and this approach is significantly more cost-effective than the use of the risk-factor approach. one research paper calculated an expected net benefit to the uk government of such an approach of around £37 million a year, compared with the current rcog approach. it has been reported that iap does to not prevent all cases of gbs-eod; its efficacy is estimated at 80%. the risk-based prevention strategy does not prevent about 33% of cases with no risk factors. up to 90% of cases of gbs-eod would be preventable if iap were offered to all gbs carriers identified by universal screening late in pregnancy, plus to the mothers in higher risk situations. where insufficient intravenous antibiotics are given before delivery, the baby may be given antibiotics immediately after birth, although evidence is inconclusive as to whether this practice is effective or not. falling incidence of eod and lod gbs disease in us-cdc home births and water birth home births are becoming increasingly popular in the uk. recommendations for preventing gbs infections in newborns are the same for home births as for hospital births. around 25% of women having home births probably carry gbs in their vaginas at delivery without knowing, and it could be difficult to follow correctly the recommendations of iap and to deal with the risk of a severe allergic reaction to the antibiotics outside of a hospital setting. the rcog and the acog guidelines suggest that birth in a pool is not contraindicated for gbs carriers who have been offered the appropriate iap if no other contraindications to water immersion are present screening for colonization approximately 10–30% of women are colonized with gbs during pregnancy. nevertheless, during pregnancy, colonization can be temporary, intermittent, or continual. because the gbs colonization status of women can change during pregnancy, only cultures carried out ≤5 weeks before delivery predict quite accurately the gbs carrier status at delivery. in contrast, if the prenatal culture is carried out more than 5 weeks before delivery, it is unreliable for accurately predicting the gbs carrier status at delivery. because of that, testing for gbs colonization in pregnant women is recommended by the cdc at 36–37 weeks of gestation. it is important to note that the acog now recommends performing universal gbs screening between 36 and 37 weeks of gestation. this new recommendation provides a 5-week window for valid culture results that includes births that occur up to a gestational age of at least 41 weeks. the clinical samples recommended for culture of gbs are swabs collected from the lower vagina and rectum through the external anal sphincter.vaginal-rectal samples should be collected using a flocked swab preferably. compared with fiber swab, these swabs releases samples and microorganisms more efficiently than conventional fiber swabs. the sample should be collected swabbing the lower vagina (vaginal introitus) followed by the rectum (i.e., inserting the swab through the anal sphincter) using the same swab or two different swabs. cervical, perianal, perirectal, or perineal specimens are not acceptable, and a speculum should not be used for sample collection. samples can be taken by healthcare professionals, or by the mother herself with appropriate instruction. instructions for the collection of a genital swab for the detection of gbs following the recommendations of the cdc, these swabs should be placed into a non-nutritive transport medium. when feasible, specimens should be refrigerated and sent to the laboratory as soon as possible. appropriate transport systems are commercially available, and in these transport media, gbs can remain viable for several days at room temperature. however, the recovery of gbs declines over one to four days, especially at elevated temperatures, which can lead to false-negative results. additional and early method of screening the us preventive services task force and acog recommend routine urine screening for all pregnant women early in their pregnancies, even in the absence of urinary symptoms, in order to detect asymptomatic bacteriuria. as with utis, any asymptomatic bacteriuria (not just gbs) cases with high cfu/ml values have also been shown to induce pyelonephritis, low birth weight, and preterm deliveries. therefore, treatment of these asymptomatic cases at the time of diagnosis with antibiotics is just as important as treating symptomatic utis in pregnancy in order to reduce these risks. in addition to cases of gbs uti and asymptomatic gbs bacteriuria with high cfu/ml, pregnant women with asymptomatic gbs bacteriuria, even with low cfu/ml counts at any time during the pregnancy, should receive iap during birth to protect the newborn, regardless of the results of the recto-vaginal screen later in pregnancy. this is because gbs bacteriuria, even asymptomatic, at any cfu/ml is an indication of heavy ano-genital colonization. culture methods samples (vaginal, rectal, or vaginorectal swabs) should be inoculated into a selective enrichment broth, (todd hewitt broth with selective antibiotics, enrichment culture). this involves growing the samples in a selective enriched broth medium to improve the viability of the gbs and simultaneously impairing the growth of other naturally occurring bacteria.appropriate enrichment broths, commercially available, are todd-hewitt with gentamicin and nalidixic acid, or with colistin and nalidixic acid (lim broth). after incubation (18–24 hours, 35-37 °c), the enrichment broth is subcultured overnight in blood agar plates and gbs-like colonies (big colonies, 3-4 millimeters diameter, surrounded by narrow zone of hemolysis) are identified by the camp test or using latex agglutination with gbs antiserum or maldi-tof. in the uk, this is the method described by the public health england's uk standards for microbiology investigations after incubation, the enrichment broth can also be subcultured to granada medium agar where gbs grows as pinkish-red colonies or to chromogenic agars, where gbs grows as coloured colonies. nevertheless, gbs-like colonies that develop in chromogenic media should be confirmed as gbs using additional reliable tests to avoid mis-identification. inoculating directly the vaginal and rectal swabs or the vaginorectal swab in a plate of an appropriate culture medium (blood agar, granada medium or chromogenic media) is also possible. however, this method (bypassing the selective enrichment broth step) can lead to some false-negative results, and this approach should be taken only in addition to, and not instead of, inoculation into selective broth. today, in the uk, the detection of gbs colonization using the enrichment broth technique is not offered from most laboratories serving the nhs. however, the implementation of this test seems to be a viable option. at present, culture for gbs (using enriched culture medium) at 35–37 weeks to define an at-risk group of women appears to be the most cost-effective strategy. the charitable organization group b strep support have published a list of hospitals in the uk that offer the detection of gbs using the enrichment broth culture method (enrichment culture medium, ecm). this test is also available privately from around £35 per test for a home-testing pack, and it is offered by private clinics. the test is also available privately, for a uk-wide postal service. point-of-care testing, poct no current culture-based test is both accurate enough and fast enough to be recommended for detecting gbs once labour starts. plating of swab samples requires time for the bacteria to grow, meaning that this is unsuitable to be used as an intrapartum point-of-care test (poct or bedside testing). alternative methods to detect gbs in clinical samples (as vaginorectal swabs) rapidly have been developed, such are the methods based on nucleic acid amplification tests, such as polymerase chain reaction (pcr) tests, and dna hybridization probes. these tests can also be used to detect gbs directly from broth media, after the enrichment step, avoiding the subculture of the incubated enrichment broth to an appropriate agar plate. testing women for gbs colonization using vaginal or rectal swabs at 35–37 weeks of gestation and culturing them in an enriched media is not as rapid as a pcr test that would check whether the pregnant woman is carrying gbs at delivery. pcr tests would allow starting iap on admission to the labour ward in those women for whom it is not known if they are gbs carriers. pcr testing for gbs carriage could, in the future, be sufficiently accurate to guide iap. however, the pcr technology to detect gbs must be improved and simplified to make the method cost-effective and fully useful as a point-of-care test. these tests still cannot replace antenatal culture for the accurate detection of gbs. poct for detection of gbs carriers requires additionally that maternity units should provide 24/7 laboratory means required to perform rapid testing. nevertheless, point-of-care testing may be used for women who present in labor with an unknown gbs status and without risk factors for ascertaining the use of iap. missed opportunities of prevention the important factors for successful prevention of gbs-eod using iap and the universal screening approach are: reach most pregnant women for antenatal screens proper sample collection using an appropriate procedure for detecting gbs administering a correct iap to gbs carriers most cases of gbs-eod occur in term infants born to mothers who screened negative for gbs colonization and in preterm infants born to mothers who were not screened, though some false-negative results observed in the gbs screening tests can be due to the test limitations and to the acquisition of gbs between the time of screening and delivery. these data show that improvements in specimen collection and processing methods for detecting gbs are still necessary in some settings. false-negative screening test, along with failure to receive iap in women delivering preterm with unknown gbs colonization status, and the administration of inappropriate iap agents to penicillin-allergic women account for most missed opportunities for prevention of cases of gbs-eod. gbs-eod infections presented in infants whose mothers had been screened as gbs culture-negative are particularly worrying, and may be caused by incorrect sample collection, delay in processing the samples, incorrect laboratory techniques, recent antibiotic use, or gbs colonization after the screening was carried out.