Streptococcus agalactiae colonization in pregnant women: which culture-based detection method is best?

Introduction. Rectovaginal colonization with Streptococcus agalactiae in pregnant women is a risk factor for invasive infections of the newborns. Various culture-based approaches are available (selective vs. non-selective media±enrichment).Hypothesis/Gap Statement. Comprehensive head-to-head comparisons of culture-based methods for S. agalactiae screening in pregnant women are largely missing.Aim. We compared the test accuracy of six culture-based approaches for the detection of S. agalactiae.Methodology. We performed a cross-sectional study on vaginal or rectovaginal swabs (in liquid Amies medium) from pregnant women. Samples were analysed in parallel in six study arms using (i) colistin nalidixic acid (CNA) agar, (ii) chromogenic S. agalactiae selective agar, (iii) CNA agar after enrichment in thioglycolate broth, (iv) chromogenic agar after enrichment in thioglycolate broth, (v) CNA agar after enrichment in Todd-Hewitt broth and (vi) chromogenic agar after enrichment in Todd-Hewitt broth. The test accuracy was calculated for each study arm using a composite reference standard.Results. In total, 244 pregnant women were included (median age: 32 years, median weeks of pregnancy: 28 3/7). Positivity was comparable between approaches with direct inoculation on solid media (study arms i and ii, 15.6-18.0%) and with primary enrichment (study arms iii-vi, 17.6-18.9%). Each study arm had a specificity of 100%, while the sensitivity varied between 81% (CNA agar alone, study arm i) and 98% (Todd-Hewitt broth for enrichment followed by subculture on chromogenic agar, study arm vi).Conclusion. Enrichment in Todd-Hewitt broth followed by subculture on chromogenic agar had the highest sensitivity (98%, specificity: 100%) for the detection of S. agalactiae in pregnant women.