Design and internal validation of an obstetric early warning score: secondary analysis of the Intensive Care National Audit and Research Centre Case Mix Programme database

Abstract

SummaryWe designed and internally validated an aggregate weighted early warning scoring system specific to the obstetric population that has the potential for use in the ward environment. Direct obstetric admissions from the Intensive Care National Audit and Research Centre's Case Mix Programme Database were randomly allocated to model development (n = 2240) or validation (n = 2200) sets. Physiological variables collected during the first 24 h of critical care admission were analysed. Logistic regression analysis for mortality in the model development set was initially used to create a statistically based early warning score. The statistical score was then modified to create a clinically acceptable early warning score. Important features of this clinical obstetric early warning score are that the variables are weighted according to their statistical importance, a surrogate for the F I O 2 /P a O 2 relationship is included, conscious level is assessed using a simplified alert/not alert variable, and the score, trigger thresholds and response are consistent with the new non-obstetric National Early Warning Score system. The statistical and clinical early warning scores were internally validated using the validation set. The area under the receiver operating characteristic curve was 0.995 (95% CI 0.992-0.998) for the statistical score and 0.957 (95% CI 0.923-0.991) for the clinical score. Pre-existing empirically designed early warning scores were also validated in the same way for comparison. The area under the receiver operating characteristic curve was 0.955 (95% CI 0.922-0.988) for Swanton et al. 's Modified Early Obstetric Warning System, for the obstetric early warning score suggested in the 2003-2005 Report on Confidential Enquiries into Maternal Deaths in the UK, and 0.973 (95% CI 0.957-0.989) for the non-obstetric National Early Warning Score. This highlights that the new clinical obstetric early warning score has an excellent ability to discriminate survivors from non-survivors in this critical care data set. Further work is needed to validate our new clinical early warning score externally in the obstetric ward environment.

A national survey of obstetric early warning systems in the United Kingdom: five years on

“…However, staffing pressures and lack of teaching and training were the two most common barriers. There was general consensus regarding the most important physiological parameters to be used in any EWS and these agreed with Carle et al and NEWS [12,13]. Continuing education, training and audit are essential to improve the uptake and utility of obstetric EWS, so that it may make a useful contribution towards the reduction of maternal morbidity and mortality.…”

“…However, they did not recommend its use in obstetrics as maternal physiology is different from the non-pregnant state. The first attempt to validate an obstetric EWS internally was based on physiological parameters derived from obstetric admissions requiring intensive care [13]. However, this EWS has not been validated on an obstetric ward, or for any outcome other than death on the intensive care unit [14].…”

“…However, this EWS has not been validated on an obstetric ward, or for any outcome other than death on the intensive care unit [14]. The majority of the obstetric EWS currently used are singleparameter, based on the CEMD-recommended chart [7], in contrast to the aggregate-weighted charts used by the National Early Warning Score or the obstetric EWS proposed by Carle et al [13]. The single-parameter systems are based on the premise that the occurrence of just one abnormal physiological parameter can be used immediately to trigger a call for help.…”

A national cross sectional survey of heads of midwifery services of uptake, benefits and barriers to use of obstetric early warning systems (EWS) by midwives

“…For example, NICE guidance (2006,2007) defined maternal pyrexia as ≥38°C or ≥37.5°C on two occasions two hours apart. Carle et al (2013) included an initial cut-off temperature of 38°C based on systemic inflammatory response criteria of obstetric patients admitted to critical care, while in the EWS charts reviewed for this study, the upper limit was 37.9°C. In addition a range of scoring systems were used, some quite complex.…”

“…The Royal College of Physicians (RCP) recommends use of a National Early Warning Score (NEWS) to standardise assessment of acute-illness severity, but does not recommend its use in children (ie ≤16 years old) or women who are pregnant because the physiological response to acute illness can be modified in children and by pregnancy (RCP 2012). Carle et al (2013) designed and internally validated an aggregateweighted obstetric EWS based on maternal mortality using secondary analysis of physiological data collected during the first 24 hours of admission of maternity patients to UK-based critical care units. Logistic regression analysis for mortality in the model development set informed a statistically based risk model for admissions, which was modified with addition of other physiological variables to create a clinical obstetric EWS.…”