title: An obstetric sphincter injury risk identification system (OSIRIS): is this a clinically useful tool?
abstract: OBJECTIVE
  To establish the contribution of maternal, fetal and intrapartum factors to the risk of incidence of obstetric anal sphincter injuries (OASIS) and assess the feasibility of an OASIS risk prediction model based on variables available to clinicians prior to birth.
  
  
  METHODS
  This was a population-based, retrospective cohort study using single-site data from the birth database of Aarhus University Hospital, Denmark. The participants were all women who had a singleton vaginal birth during the period 1989 to 2006. Univariate and multivariate logistic regression analyses were performed using multiple imputations for missing data and internally validated using bootstrap methods. The main outcome measures were the contributions of maternal, fetal and intrapartum events to the incidence of OASIS.
  
  
  RESULTS
  A total of 71,469 women met the inclusion criteria, of whom 1,754 (2.45 %) sustained OASIS. In the multivariate analysis of variables known prior to birth, maternal age 20 - 30 years (OR 1.65, 95 % CI 1.44 - 1.89) and ≥30 years (OR 1.60, 95 % CI 1.39 - 1.85), occipitoposterior fetal position (OR 1.34, 95 % CI 1.06 - 1.70), induction/augmentation of labour (OR 1.46, 95 % CI 1.32 - 1.62), and suspected macrosomia (OR 2.20, 95 % CI 1.97 - 2.45) were independent significant predictors of OASIS, with increasing parity conferring a significant protective effect. The 'prebirth variable' model showed a 95 % sensitivity and a 24 % specificity in predicting OASIS with 1 % probability, and a 3 % sensitivity and a 99 % specificity in predicting OASIS with a 10 % probability.
  
  
  CONCLUSIONS
  Our model identified several significant OASIS risk factors that are known prior to actual birth. The prognostic model shows potential for ruling out OASIS (high sensitivity with a low risk cut-off value), but is not useful for ruling in the event.
authors: Institute of Metabolism and Systems Research, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK. webbyali@blueyonder.co.uk.|Delivery Suite, Birmingham Women's NHS Foundation Trust, Mindelsohn Way, Edgbaston, Birmingham, B15 2TG, UK. webbyali@blueyonder.co.uk.
authors: Sara S
authors: SS
authors: Webb
authors: Institute of Applied Health Research, University of Birmingham, Birmingham, UK.
authors: Karla
authors: K
authors: Hemming
authors: School of surgery, North Western Deanery, Manchester, UK.
authors: Madhi Y
authors: MY
authors: Khalfaoui
authors: Perinatal Epidemiology Research Unit, Aarhus University Hospital, Aarhus, Denmark.
authors: Tine Brink
authors: TB
authors: Henriksen
authors: Perinatal Epidemiology Research Unit, Aarhus University Hospital, Aarhus, Denmark.
authors: Sara
authors: S
authors: Kindberg
authors: Perinatal Epidemiology Research Unit, Aarhus University Hospital, Aarhus, Denmark.
authors: Stine
authors: S
authors: Stensgaard
authors: Staffordshire University, Stafford, UK.
authors: Christine
authors: C
authors: Kettle
authors: Institute of Metabolism and Systems Research, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK.|Delivery Suite, Birmingham Women's NHS Foundation Trust, Mindelsohn Way, Edgbaston, Birmingham, B15 2TG, UK.
authors: Khaled M K
authors: KM
authors: Ismail
mesh_terms: D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D001724:Birth Weight; D036861:Delivery, Obstetric / Q000009:adverse effects*; D003718:Denmark / Q000453:epidemiology; D005260:Female; D006801:Humans; D015994:Incidence; D007231:Infant, Newborn; D022125:Lacerations / Q000453:epidemiology*; D016015:Logistic Models; D007744:Obstetric Labor Complications / Q000453:epidemiology*; D011237:Predictive Value of Tests; D011247:Pregnancy; D012189:Retrospective Studies; D018570:Risk Assessment; D012307:Risk Factors; D018709:Statistics, Nonparametric
keywords: Cohort study; OASIS; Predictor variables; Prognostic model; Risk factors