Based on data from a preliminary analysis of the differences between measured hemoglobin mass loss and estimated hemoglobin loss, a sample of 100 cases was considered necessary. This calculation was made using the method proposed by Lu et al.30 to determine the sample size required to meet the maximum compliance limits of 95% of the agreement (LOA) for ±71 g in the Bland-Altman analysis. The sample size was calculated with a statistical performance of 90% and a meaning level of 0.05 (two-tails). To correctly assess the accuracy of the hemoglobin mass loss formula, two formulas for estimating blood volume loss were tested. In the blood loss estimation formulas currently used, blood loss (in units of volume) is calculated by multiplying the perioperative difference in hemoglobin (or hematocrite) by the patient`s estimated blood volume. The main difference between these formulas is that the perioperative difference in hemoglobin (or hematocrite) is adjusted by different mathematical functions that aim to correct the expected dilution effect caused by blood loss7. Nevertheless, it is unlikely that the hypothesis of a predictable dilution effect is correct in the perioperiarary approach, since significant inter- and intra-individual variations in blood volume (specifically plasma volume) have been demonstrated during this period, due to the complexity of physiological mechanisms of volume replacement 9,19-21. In addition, fluid administration, pathological movements and anesthesia also affect the state of plasma volume9,10. As plasma volume varies with intra- and perioperative periods, blood volume moves between a large number of hemodérie states13. As blood loss usually occurs at different times during the operation, blood lost during this period may have different dilution rates and therefore different hemoglobin levels14. Therefore, the loss of blood volume would not be properly reflected in the hematological parameters used in blood loss estimation formulas. There is evidence that midwives are relatively accurate in estimating blood loss.

Kavle et al.[14] indicated that nurse-midwives could estimate the ability of nurses and midwives to estimate blood loss during childbirth as exactly within 5 ml after a laboratory evaluation; However, the greater the blood loss, the more imprecise the estimate is due to an underestimation or overestimation of the loss. In the case of loss of > 200 ml, the average difference from laboratory results was 62 ml, either underestimated or overestimated. Glover[15] also reported accuracy in estimating blood loss in midwives during a simulated birth; However, errors increased when the blood loss > was 600 ml. Similarly, Budny et al.[16] reported a strong positive correlation between calculated estimates of blood loss and blood loss of youth and chief surgeons and chief anaesthetists after cremation surgery. Withanathantrige M, Goonewardene M, Dandeniya R, Gunatilake P, Gamage S. Comparison of four methods of estimating blood loss after caesarean section. Int J Gynaecol Obstet. 2016;135:51-5. Thurer RL, Castro JM, Javidroozi M, Burton K, Bernal NP. Accurate measurement of intraoperative blood loss improves prediction of postoperative hemoglobin levelsMeur measuring intraoperative blood loss improves the prediction of postoperative hemoglobin levels. J Anesth Clin Res.

2017. estimate of hemoglobin mass loss (mHbEBL): Estimate of mass hemoglobin loss calculated using the hemoglobin mass loss formula described above.