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In Reply:
We thank the authors for their response to our publication.1 The main intent of our study was to emphasize the need for a more updated outlook on postpartum hemorrhage definition. We hypothesized that a single value of 500 mL does not represent the same change in blood volume in different women. To prove that, we used body surface area as our basis for measuring total blood volume.
Physiologic changes in pregnancy affect body weight, body composition, body surface area, and body mass index. Multiple formulae have been proposed to calculate total blood volume based on one or more of the above parameters.2,3 In several of these studies, including the one by Nadler et al4 (the formula used in the study by Milosevic et al, quoted in your letter5), the difference in total blood volume as calculated by body surface area or body mass index is relatively similar. However, given the complexity of pregnancy changes, we chose body surface area based on the study by Wang et al6: “BSA [body surface area] correlates with metabolic rate, cardiac output, oxygen consumption, renal plasma flow, urea clearance, glomerular filtration rate and organ size better than physical measurements such as weight.” In this study, Wang et al point out that the best formula to predict body surface area in pregnant women is the Dubois formula as adopted by us.
In response to the author's query regarding obstetric hemorrhage being “defined based on one of the trauma shock classifications,” our intent was not this at all. We used the trauma shock classification as an example to show the different percentages of total blood volume that varied with different body surface areas, proving our point again that, rather than using a single value of 500 mL, a range is much needed.
We agree with the authors that estimated blood loss is very subjective. We used this only as an example to show the variations in the total blood volume but not as the basis of what to be expected as the norm. We would like to reference Table 2 in our study,1 where the total blood volume and 5, 10, and 15% volumes were calculated for different body surface area quintiles. That was the main purpose of this article. This was also the reason for not including blood transfusions.
Our future direction would be to validate this with quantified blood loss and assess clinical changes at different percentages of total blood volume lost and blood transfusions. We did group all our pregnant patients together; however, this should not have changed our outcome, because the measurements were all taken on admission and would have been reflective of the patient's current gestational period.
We agree with the authors regarding the existence of multiple confounding factors such as multiple gestation, which was the reason we did not include that group in our study.
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