Statistical analysis
anaemia
REGRESSION, POSTPARTUM HAEMORRHAGE, ANAEMIA
We reported the cumulative incidence (risk) of postpartum haemorrhage over the observation period (24 h after birth). We then used multivariable logistic regression to examine the association between prebirth haemoglobin concentration and postpartum haemorrhage. The 10 g/L decrease in haemoglobin was obtained as an explanatory variable in the multivariable logistic regression model. To account for clustering, we included a random effect at the site level and a fixed effect for country. The same confounding factors were used in the models for each of the three definitions for postpartum haemorrhage. From the obstetric literature, we identified factors likely to be associated with maternal anaemia and the risk of postpartum haemorrhage. We described our causal assumptions using a directed acyclic graph (We examined the association between haemoglobin concentration and postpartum haemorrhage stratified by the presence or absence of other risk factors for postpartum haemorrhage. We created an indicator variable for having one or more risk factors. We used crude logistic regression to estimate the association between haemoglobin and clinical postpartum haemorrhage. We added polynomial terms for haemoglobin and used likelihood ratio tests to see if this improved the model fit. We used a threshold of p<0·05. We then added interaction terms for the indicator variable and all haemoglobin terms to our model. We checked if interaction terms improved fit in a likelihood ratio test with a threshold of p<0·05.We also corrected for haemoglobin measurement error. The COMPARE study validated haemoglobin measurements obtained using capillary HemoCue samples against a laboratory gold standard in 5724 women who were aged 18 years or older and eligible to give blood through the UK's National Health Service donation service.