Prevention of postpartum haemorrhage: a distributional approach for analysis

Abstract

Background: There is empirical evidence that measured postpartum blood loss has a lognormal distribution. This feature can be used to analyze events of the type 'blood loss greater than a certain cutoff point' using a lognormal approach, which takes into account all the quantitative observations, as opposed to dichotomizing the variable blood loss volume into two categories. This lognormal approach uses all the information contained in the data and is expected to provide more efficient estimates of proportions and relative risk when comparing treatments to prevent postpartum haemorrhage. As a consequence, sample size can be reduced in clinical trials, while keeping the statistical precision requirements.

Methods: The authors illustrate how a lognormal approach can be used in this situation, using data from a clinical trial and the event 'blood loss greater than 1000 mL'.

Results: Estimates of the proportions of this event for each treatment, and relative risks obtained with this method are presented and compared with the standard estimates obtained by dichotomizing measured blood loss volume. An example of how the blood loss distributions of two treatments can be compared is also presented. Different scenarios of the sample size needed to compare two treatments or interventions are presented to illustrate how with the lognormal approach the size of a clinical trial can be reduced.

Conclusions: A distributional approach for postpartum blood loss using the lognormal distribution fitted to the data results in more precise estimates of risks of events and relative risks, compared to the use of binomial proportions of events. It also results in reduced required sample size for clinical trials.

Trial registration: This paper reports a secondary analysis for a trial that was registered at clinicaltrials.gov ( NCT00781066 ).

Keywords: Lognormal distribution; Postpartum blood loss; Postpartum haemorrhage; Severe postpartum haemorrhage.

Conflict of interest statement

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Competing interests

The authors declare not to have any competing interests.

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Figures

Fig. 1

Frequency distribution of blood loss volume (mL) by treatment, Althabe et al. trial [1]

Fig. 2

Probabilistic plot of the THLN fitted curve (red line) with pointwise 95% confidence band (red area), showing the data points (black dots) with their 95% confidence intervals (blue lines), Althabe et al. trial, hands-off treatment [1]

Fig. 3

THLN fitted curve (red line) with pointwise 95% confidence band (red area), the empirical survival function (black dots) with the pointwise 95% confidence intervals (blue lines), Althabe et al. trial, hands-off treatment [1]

Fig. 4

Distributions of blood loss for the two treatments, Hands-off (red) and CCT (blue), side by side on a lognormal probability plot, showing data points (dots), fitted lognormal lines (continued full lines), and 95% confidence bands (shaded areas), Althabe et al. trial [1]