Ascertaining severe perineal trauma and associated risk factors by comparing birth data with multiple sources

Population data are often used to monitor severe perineal trauma trends and investigate risk factors. Within New South Wales (NSW), two different datasets can be used, the Perinatal Data Collection ('birth' data) or a linked dataset combining birth data with the Admitted Patient Data Collection ('hospital' data). Severe perineal trauma can be ascertained by birth data alone, or by hospital International Classification of Diseases Australian Modification (ICD-10-AM) diagnosis and procedure coding in the linked dataset. The aim of this study was to compare rates and risk factors for severe perineal trauma using birth data alone versus using linked data.

The study population consisted of all vaginal births in NSW between 2001 and 2011. Perineal injury coding in birth data was revised in 2006, so data were analysed separately for 2001-06 and 2006-11. Rates of severe perineal injury over time were compared in birth data alone versus linked data. Kappa and agreement statistics were calculated. Risk factor distributions (maternal age, primiparity, instrumental birth, birthweight ≥4 kg, Asian country of birth and episiotomy) were compared between women with severe perineal trauma identified by birth data alone, and those identified by linked data. Multivariable logistic regression was used to calculate the adjusted odds ratios (aORs) of severe perineal trauma.

Among 697 202 women with vaginal births, 2.1% were identified with severe perineal trauma by birth data alone, and 2.6% by linked data. The rate discrepancy was higher among earlier data (1.7% for birth data, 2.4% for linked data). Kappa for earlier data was 0.78 (95% CI 0.78, 0.79), and 0.89 (95% CI 0.89, 0.89) for more recent data. With the exception of episiotomy, differences in risk factor distributions were small, with similar aORs. The aOR of severe perineal trauma for episiotomy was higher using linked data (1.33, 95% CI 1.27, 1.40) compared with birth data (1.02, 95% CI 0.97, 1.08).

Although discrepancies in ascertainment of severe perineal trauma improved after revision of birth data coding in 2006, higher ascertainment by linked data was still evident for recent data. There were also higher risk estimates of severe perineal trauma with episiotomy by linked data than by birth data.