The authors looked at birth records on 254 infants who had GBS disease and 7,437 infants who did not.
Between 1998-1999 and 2003-2004, screening rates jumped from just over 48 percent to 85 percent. This resulted in almost 32 percent of infants being exposed to antibiotics during delivery in the later timeframe, up from about 27 percent in the late 1990s.
Overall, there were 0.32 cases of GBS per 1,000 live births. Infants born premature had a higher incidence of GBS than infants born full-term (0.73 vs. 0.26 cases per 1,000 live births). Yet, most of the cases occurred in full-term infants.
Just over 13 percent of full-term GBS cases occurred in babies of mothers who had not been screened, whereas 61.4 percent occurred in babies of mothers who had actually tested negative for GBS at 35 to 37 weeks of pregnancy.
"Although the antenatal screening strategy has been very successful, we are also exploring ways that we can do even better, and one of them is understanding the issues around false-negatives," Van Dyke said. "While some false-negatives are expected because of the limits of the culture test, we were able to use our study design to show that we observed more false negatives than would be expected."
The term "false-negative" may be a bit of a misnomer, however, Greenberg pointed out.
"If screening is done at 35 weeks gestation, it is possible for a woman who is negative -- i.e., not a carrier -- to become a carrier by the time she delivers," Greenberg said. "Then there's a higher risk that she will pass on group B strep to her infant. Almost all the patients who we see now with group B strep are babies who are born to mothers who had 'false negative' ki
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