Evaluation of a loop mediated isothermal amplification (LAMP) assay for the detection of group B streptococci

BACKGROUND GBS are still the leading cause of severe neonatal disease. Screening for vaginal-rectal GBS colonization in pregnant women at 35-37 weeks of gestation, followed by intrapartum antibioprophylaxis to colonized women has proven to be the most effective strategy for prevention of perinatal GBS disease. Despite GBS improved culture method, predictive values (PV) of prenatal screening for GBS colonization at delivery remain limited and contribute to ongoing disease. Switching from time-consuming cultures to a rapid molecular based assay for prenatal screening could be considered: a new assay, the illumigene® Group B Streptococcus (illuB) from Meridian Bioscience, based on LAMP technology is awaited to improve antenatal screening sensitivity.
OBJECTIVE To evaluate the illuB performed on culture obtained by 18-24 hours incubation of vaginal-rectal swab in Lim broth versus the reference culture for GBS screening. To determine if and how the illuB may be integrated in the current recommended screening strategy.
METHODS Vaginal-rectal specimens collected from 242 pregnant women, were sent to and processed by the labs of the University Hospital of Liege and Hospital Sint Lucas of Gent to determine the status of GBS colonization by the reference culture method for prenatal GBS screening and by illuB. Cultures were performed by direct plating onto Granada agar and inoculation in a selective enrichment Lim broth subsequently subcultured after overnight incubation onto Granada and StrepB Select agar. All illuB were performed from the same incubated Lim broth. Discrepant results were controlled by a PCR assay detecting a gene different from the target of the illuB.
RESULTS GBS were recovered in culture from 20.7% of the specimens. By comparison with culture, sensitivity and specificity (S, Sp) of the illuB for identifying GBS colonization were 90% and 98.9%. The positive and negative PV (PPV,NPV) were 95.7% and 97.4% respectively. Among the discrepancies, the 2 illuB positive/culture negative results were also tested positive by the control PCR assay, confirming the presence of GBS DNA even if GBS were not cultured from these specimens. Overall for GBS detection, Sp and PPV were 100%. Out of the 5 false negative illuB, 3 matched cultures with very rare GBS meaning that GBS could have been absent in the aliquots used for illuB. Therefore, S and NPV of the illuB for the detection of GBS are probably higher than 90.4% and 97.4%.
CONCLUSION The illuB demonstrated S and Sp close to those of the reference culture method and did not significantly improve the GBS detection. The higher cost of such molecular test may be an obstacle to its wide use as there is not a net benefice when compared with culture method; but a shorter turnaround time and limited hands on time could balance positively the decision. Additional clinical trials are needed.