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We are grateful to the authors of “Outcome of a screening programme for the prevention of neonatal invasive early-onset group B Streptococcus infection in a UK maternity unit:
an observational study” for publishing the results of the screening programme for GBS carriage at Northwick Park Hospital.1 The impetus behind the programme was clearly driven by concern for the health of newborn babies and enthusiasm to reduce the rate of EOGBS in the hospital’s population.
However, the national policy on culture based screening at 35-37 weeks gestation is that this should not be offered. In large part this is informed by concern about the screening test’s inability to reliably distinguish between women whose babies would be affected by EOGBS and those whose babies would not. The consequence of this is a high rate of overdiagnosis and subsequent overtreatment. The Northwick Park experience provides an insight into this which was not brought out in the paper.
The paper reports an EOGBS rate of 0.99 / 1000 deliveries prior to screening and a GBS carriage rate of 29% in the population. With 9098 live births in the study period 9 cases of EOGBS in approximately 2600 carriers would be expected. Screening at 35 – 37 weeks aims to identify these carriers and offer IAP to reduce the risk of EOGBS. From what is presented in the paper regarding transmission rates, and elsewhere regarding test accuracy,2,3 between 60% and 80% of these carriers would be eligible for IAP wh...
The paper reports an EOGBS rate of 0.99 / 1000 deliveries prior to screening and a GBS carriage rate of 29% in the population. With 9098 live births in the study period 9 cases of EOGBS in approximately 2600 carriers would be expected. Screening at 35 – 37 weeks aims to identify these carriers and offer IAP to reduce the risk of EOGBS. From what is presented in the paper regarding transmission rates, and elsewhere regarding test accuracy,2,3 between 60% and 80% of these carriers would be eligible for IAP when they were no longer carriers or would not transmit GBS to the baby during labour. In the remaining group between 1% and 2% would expect to be affected by EOGBS even in this population with a higher incidence than the national mean. So the overwhelming majority of women treated would be overdiagnosed and given unnecessary intravenous antibiotics during labour.
The recent UK NSC evidence review in this area highlighted that that there has been inadequate exploration of the potential harms associated with this strategy.4 With such high rates of overdiagnosis, a better understanding of the balance of benefit and harm is needed, particularly in the era of anti-microbial resistance.
It can be estimated from the report that, in practice, about 45% of the total number of carriers received IAP during the study period. If those receiving IAP for an ineffective duration are also considered to be untreated this falls to about 35%. This not only draws attention to the complexities and challenges likely to be encountered in delivering a programme on this scale, but it also draws attention to some methodological issues in the paper.
The possibility that factors beyond the screening programme accounted for, or contributed to, the reduction in EOGBS is acknowledged. A number of maternal characteristics were reported to be significantly different between the screening and risk-based groups, but it was unclear whether an analysis adjusting for potential confounders was undertaken and, if so, why it was not reported in the paper. In addition, the comparison between EOGBS incidence in the screening and risk based periods was not statistically significant. It is unclear on that basis why the conclusion states that the screening programme is associated with a fall in infection rates. Similarly the additional comparison between a selective subgroup of women from the screening period (only those who accepted screening) and all women during the risk based period reported a confidence interval of 0.02 to 1.18 for the risk ratio. While this is also not significant, the p-value is <0.05. We cannot establish from the methods section what caused this incongruence, and whether it is statistically significant. This additional comparison of outcomes in a selection of one population with outcomes from the whole population from the earlier period may also be inherently biased as we know that people who accept screening are systematically different to those who do not.
As a final point for information, the paper says that the UK NSC ‘advocates’ a risk based prevention strategy. This is incorrect. The Royal College of Obstetricians and Gynaecologists and NICE have produced guidance relating to a risk based strategy. The UK NSC has only made recommendations relating to a screening programme. In countries where screening has been implemented, such as the USA, this has been added to the risk based strategy but has not replaced it as an alternative.5
Whilst this study is a valuable case study of the practicalities of implementing screening in the UK context, uncertainty on the programme’s impact may not have been adequately explored. More importantly, the study does not fill the evidence gaps identified in the UK NSC review, including the potential harms of IAP in labour, whether there are benefits of screening over a risk-based strategy, and the need to identify a more accurate test to predict which babies will develop EOGBS. This may limit confidence in the paper’s recommendation that other units should follow suit by introducing screening.
1. Gopal Rao G, Nartey G, McAree T, et al. Outcome of a screening programme for the prevention of neonatal invasive early-onset group B Streptococcus infection in a UK maternity unit: an observational study. BMJ Open. 2017;7:e014634.
2. Colbourn T, Asseburg C, Bojke L, et al. Prenatal screening and treatment strategies to prevent group B streptococcal and other bacterial infections in early infancy: cost-effectiveness and expected value of information analyses. Health Technol Assess. 2007;11:1-226, iii.
3. Valkenburg-van den Berg AW, Houtman-Roelofsen RL, Oostvogel PM, et al. Timing of group B streptococcus screening in pregnancy: a systematic review. Gynecol Obstet Invest. 2010;69:174-183.
4. Seedat F, Taylor-Phillips S, Geppert J, et al. Universal antenatal culture-based screening for maternal Group B Streptococcus (GBS) carriage to prevent early-onset GBS disease. United Kingdom: UK National Screening Committee, Public Health England; 2016.
5. Verani JR, McGee L, Schrag SJ. Prevention of perinatal group B streptococcal disease--revised guidelines from CDC, 2010. MMWR Recomm Rep. 2010;59:1-36.