Performance of RT-PCR in the detection of Streptococcus agalactiae in the anogenital tract of pregnant women

Introduction Infection with Group B Streptococcus (GBS) is the most frequent in the first weeks of life of a newborn. The identification of pregnant women with GBS colonization may reduce neonatal infection. Methods This cross-sectional study evaluated the performance of real-time polymerase chain reaction (RT-PCR) to detect GBS colonization in the anogenital tract of pregnant women. Anogenital swabs were collected from 266 pregnant women from December 2010 to August 2011. GBS was detected using culture (gold standard) and RT-PCR to determine sip gene expression. The presence of DNA was confirmed using betaglobin amplification, and the guanidine technique was used for DNA extraction. When results were discordant, the test was repeated using conventional PCR. The results were evaluated to determine sensitivity, specificity, positive and negative predictive values and accuracy. Results Of the 266 samples collected, 254 were adequate for analysis. Prevalence was 28.7 % using the gold standard criterion and 38.2 % using RT-PCR. The comparison of RT-PCR with culture revealed a sensitivity of 89 % (95 % CI 0.81-0.96), specificity of 82 % (95 % CI 0.76-0.87), positive predictive value of 67 % (95 % CI 0.57-0.76) and negative predictive value of 94 % (95 % CI 0.91-0.99). Conclusion Further studies using other DNA extraction techniques, targeting other GBS genes and using sample enhancement before RT-PCR should be conducted to determine whether the sensitivity and specificity recommended by the CDC may be reached using the same thermal cycler.