Should all pregnant women be offered testing for group B streptococcus?

Dear Editor,

We write with reference to the article by Walker et al titled ‘Should all pregnant women be offered testing for group B streptococcus?’, which we read with great interest.[1]

The authors highlight the risk of maternal group B streptococcus (GBS) exposure to newborns, and discuss the two main strategies utilised to identify pregnant women that need antibiotic prophylaxis to reduce early onset neonatal GBS infection. UK guidelines recommend against routine universal testing for maternal GBS and instead propose targeting antibiotic prophylaxis in labour to women who have risk factors for GBS transmission.[2] An alternative strategy, carried out in other countries, involves testing all women for GBS in late pregnancy. Each approach is associated with different risks and benefits. The article highlights ongoing/proposed randomised trials that will aid in providing further research to the current evidence base.[1]

We agree that further research is needed, but would like to highlight the issue of possible misidentification of Streptococcus pseudoporcinus as GBS. S. pseudoporcinus is a separate, recently identified Beta-haemolytic gram-positive coccus, known to colonise to female genital tract, which has biochemical characteristics similar to those of GBS. Due to these similar biochemical characteristics, S. pseudoporcinus can cause false positives in standard commercially available GBS serogrouping kits used in diagnostic laboratories. One single centre retrospective study demonstrated that 2.5% of GBS results were false positives (when comparing grouping test versus matrix-assisted laser desorption ionization (MALDITOF)), and reflected S. pseudoporcinus colonization.[4] Another prospective observational study found that S. pseudoporcinus cross-reacted with GBS across 3 different types of grouping kit, with cross-reactivity ranging from 38% to 93%.[5] Advances in micro-organism identification may change the known prevalence of S. pseudoporcinus and further research is needed to clarify the clinical significance of S. pseudoporcinus colonization, especially in regard to neonatal and maternal infections.[4, 5]

Interestingly, the UK standard microbiological investigations protocol does not mention the possibility of this misidentification.[3] In an age of antimicrobial resistance, and with the potential risks of intra-partum antibiotics to maternal and neonatal health, we suggest that we should aim to improve diagnostic accuracy if testing for GBS is increased. Thus in both the proposed randomised trials we recommend utilising methods of identification such as MALDITOF, rather than relying solely on grouping kits to improve GBS diagnosis.

References
1. Walker KF, Plumb J, Gray J et al. Should all pregnant women be offered testing for group B streptococcus? BMJ 2021;373:n882
2. Hughes RG, Brocklehurst P, Steer PJ et al on behalf of the Royal College of Obstetricians and Gynaecologists. Prevention of early-onset neonatal group B streptococcal disease. Green-top Guideline No. 36. BJOG 2017; 124:e280–e305.
3. Public Health England and NHS. UK Standards for Microbiology Investigations Identification of Streptococcus species, Enterococcus species and Morphologically Similar Organisms. https://assets.publishing.service.gov.uk/government/uploads/system/uploa.... Date Accessed: 31/05/2020.
4. Grundy M, Suwantarat N, Rubin M et al. Differentiating Streptococcus pseudoporcinus from GBS: could this have implications in pregnancy? American Journal of Obstetrics and Gynaecology 2019; 220(5): 490.e1-490.e7.
5. Suwantarat N, Grundy M, Rubin M et al. Recognition of Streptococcus pseudoporcinus Colonization in Women as a Consequence of Using Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry for Group B Streptococcus Identification. Journal of Clinical Microbiology 2015; 53(12).