Re: 'Myth: Group B streptococcal infection in pregnancy: comprehended and conquered' published in volume 16 (2011) pp 254-258

Joint synthesis of conditionally related multiple outcomes makes better use of data than separate meta-analyses

BACKGROUND

When there are structural relationships between outcomes reported in different trials, separate analyses of each outcome do not provide a single coherent analysis, which is required for decision-making. For example, trials of intrapartum anti-bacterial prophylaxis (IAP) to prevent early onset group B streptococcal (EOGBS) disease can report three treatment effects: the effect on bacterial colonisation of the newborn, the effect on EOGBS, and the effect on EOGBS conditional on newborn colonisation. These outcomes are conditionally related, or nested, in a multi-state model. This paper shows how to exploit these structural relationships, providing a single coherent synthesis of all the available data, while checking to ensure that different sources of evidence are consistent.

RESULTS

Overall, the use of IAP reduces the risk of EOGBS (RR: 0.03; 95% Credible Interval (CrI): 0.002-0.13). Most of the treatment effect is due to the prevention of colonisation in newborns of colonised mothers (RR: 0.08, 95% CrI: 0.04-0.14). Node-splitting demonstrated that the treatment effect calculated using only direct evidence was consistent with that predicted from the remaining evidence (p = 0.15). The findings accorded with previously published separate meta-analyses of the different outcomes, once these are re-analysed correctly accounting for zero cells.

CONCLUSION

Multiple outcomes should be synthesised together where possible, taking account of their structural relationships. This generates an internally coherent analysis, suitable for decision making, in which estimates of each of the treatment effects are based on all available evidence (direct and indirect). Separate meta-analyses of each outcome have none of these properties.

Group B streptococcal disease and genotypes in Australian infants

AIMS

Estimate the incidence, risk factors and clinical features of group B streptococcal (GBS) disease in Australian infants and compare bacterial genotypes between infant disease and maternal carriage.

METHODS

The Australian Paediatric Surveillance Unit conducted a study of invasive GBS disease in infants aged 0-90 days between July 2005 and June 2008. Clinical data were obtained by questionnaire. GBS isolates from affected infants and antenatal carriers were referred for genotyping.

RESULTS

One hundred fifty confirmed cases were reported: 88 early-onset (EOD; 0-6 days) and 62 late-onset disease (LOD). Based on review of selected laboratory records, they represented ∼1/3 of all cases. Estimated national EOD and LOD rates were 0.38 and 0.19/1000 live births, respectively. Admission to intensive care was common (44%), and 7% of infants died. One or more obstetric indications for intrapartum antibiotic prophylaxis (IAP) were identified in 51% of mothers; 53% of mothers were screened for GBS carriage, and screening was positive in 45%; only 25% of women with clinical or microbiological risk factors were given IAP (no significant differences between EOD and LOD groups). Distribution of GBS genotypes differed significantly between invasive and vaginal isolates: virulent serosubtype III-2/sequence type 17 was strongly associated with LOD but uncommon among EOD and vaginal isolates.

CONCLUSIONS

Estimated GBS disease rates remain relatively low despite poor predictive values of clinical and microbiological criteria for, and compliance with, IAP. Clinical and microbiological epidemiology of LOD differs significantly from that of EOD. Further reduction in infant morbidity and mortality from GBS disease will require new strategies.