False Negatives For Strep B
Now that a federal recommendation has been issued to the effect that all pregnant women should be tested for Group B strep bacteria, many more women are having the test. That's good news. However, it seems that the sudden large numbers of pregnant women having the test have had the effect of producing a high number of false negative test results. This is according to a study published in June 2009 in the New England Journal of Medicine.
The common bacteria known as Group B strep is found in the intestinal tract or in the genitals. While the bacteria pose no risk to most adults, they can cause great harm to infants who become infected during delivery. Infants who contract Group B strep during delivery can develop all sorts of complications including blood infections, hearing and vision loss, pneumonia, mental retardation, meningitis, or even death.
Group B strep infant infections are not very common, occurring as they do in one out of every 3,000 deliveries, but the Centers for Disease Control and Prevention (CDC) issued a recommendation in 2002 for all pregnant women to undergo testing since the risk for serious and heartbreaking complications does exist.
This study is the first to analyze the results of the Group B strep testing campaign. Researchers looked at data on infant Group B strep cases from 10 states. They discovered that 250 babies out of a total of 7,700 contracted the infection during delivery. The researchers compared these results to those of a similar study performed before the CDC recommendations had been issued. It was found that the rate of screening for Group B strep in pregnant women had risen from 48%-85%. Also, it was determined that the rate of infant infections from the bacteria had gone down by 27%.
Researchers had made the prediction that there would be somewhere between 44 and 86 false negative results in babies delivered full-term. They based these figures on earlier studies. In fact, the results showed that some 60% of infected infants, representing 116 cases, had been born to women who had received negative results on their Group B strep screening tests.
Researchers posit that the timing of the administration of the Group B test might have an effect on the results, since an infection can come swiftly. The tests may have been performed before the bacteria were present. The CDC's recommendation is for women to be screened between gestation weeks 35 and 37. Stephanie Schrag, a researcher with the CDC and a co-author of this study commented, "Maybe it was a true negative test, and the mother later became colonized [with Group B strep bacteria]."