In August 2012, the Standards Unit of the Health Protection Agency’s Microbiology Services Division issued an update to their UK Standards for Microbiology Investigations (SMI), Processing Swabs for Group B Streptococcal Carriage. The document has been updated for clarity and is presented in a new format, although there are no major changes to the guidance.
This SMI describes the processing of specimens from pregnant women for carriage of group B Streptococci and provides a standardised method for culture where clinicians decide to investigate specific patients. As the SMI states,
“according to local protocols, patients judged clinically to be at high risk for the development of Group B streptococcal infection may be investigated for carriage. The isolation rate of GBS from clinical specimens depends on several factors. Studies have shown that the accuracy of prenatal screening cultures for identification of GBS colonisation can be enhanced by attention to the timing of cultures, the sites swabbed and the microbiological method used for culture of organisms. Collection of swabs between 35 and 37 weeks gestation is recommended to improve the sensitivity and specificity of detection of colonisation at the time of delivery11. Optimum yield will be achieved by selective/enrichment procedures applied to swabs obtained from the vagina and the anorectum which increases the likelihood of GBS isolation by up to 30% compared with vaginal or cervical culture alone. Vaginal and rectal swabs are likely to isolate a diverse array of normal flora and use of selective enrichment broth is recommended to avoid overgrowth of other organisms.”
It remains astonishing to us at GBSS that so few NHS hospitals make this test available either to pregnant women in their care or at the request of their health professionals.