Am Fam Physician. 2005;72(6):1116
Group B streptococci (GBS) affects two in 1,000 live births, with a case fatality rate of 5 to 10 percent for early-onset neonatal sepsis. Earlier studies showed that the risk of GBS sepsis could be reduced by intrapartum antibiotic prophylaxis. Because of these findings, groups of experts recommend screening all pregnant women for GBS colonization at 35 to 37 weeks of gestation and administering intrapartum antibiotic prophylaxis to those who are colonized. The guidelines call for screening swabs to be collected from the lower third of the rectum and vagina; a study in 1977 showed that rectal cultures were positive more often than vaginal cultures. In 2002, a study showed that there was no significant difference in recovery of GBS from the rectum compared with cultures from the perianal area and from the perianal area and vagina combined. This study also indicated that three fourths of women experience pain with the rectal swab. Jamie and colleagues compared vaginal plus perianal cultures with the conventional vaginal plus rectal cultures to determine if patients could be spared the discomfort of rectal specimens without compromising the detection of GBS.
The authors studied 200 women attending a university clinic for prenatal care between July and October 2003. Specimens were collected from the vagina, rectum, and perianal area. All swabs were processed identically, and laboratory personnel were blinded to the site of sample collection.
The average age of patients was 25 years; 58 percent were white; 79 percent used Medicaid; 44 percent were married; 43 percent were nulliparous; and the average gestational age at sampling was 35 weeks. One or more positive cultures were obtained from 71 patients (35.5 percent). Vaginal samples were positive in 55 patients (27.5 percent), perianal samples in 48 patients (24 percent), and rectal samples in 50 patients (25 percent). When vaginal and perianal cultures were combined, 68 patients (34 percent) were positive. Combined rectal and vaginal culture results were positive in 67 patients (33.5 percent). Vaginal, rectal, and perianal cultures done alone each had significantly lower detection rates than the combined cultures.
The authors concluded that the combination of vaginal and perianal specimens is equivalent to vaginal plus rectal specimens in the detection of GBS. They stress the importance of screening for GBS in pregnant women, and believe the discomfort of rectal specimen collection to be unnecessary.