Perioperative blood loss: estimation of blood volume loss or haemoglobin mass loss?

Abstract

Background: Perioperative blood loss is an essential parameter in research into Patient Blood Management. However, currently there is no "gold standard" method to quantify it. Direct measurements of blood loss are considered unreliable methods, and the formulae to estimate it have proven to be significantly inaccurate. Given the need for better research tools, this study evaluated an estimation of haemoglobin mass loss as an alternative approach to estimate perioperative blood loss, and compared it to estimations based on blood volume loss.

Material and methods: We studied one hundred consecutive patients undergoing urological laparoscopic surgery. Both haemoglobin mass loss and blood volume loss were directly measured during surgery, under highly controlled conditions for a reliable direct measurement of blood loss. Three formulae were studied: 1) a haemoglobin mass loss formula, which estimated blood loss in terms of haemoglobin mass loss, 2) the López-Picado's formula and 3) an empirical volume formula that estimated blood loss in terms of blood volume loss. The empirical volume formula was developed within the study with the aim of providing the best possible estimation of blood volume loss in the studied population. The formulae were evaluated and compared by assessing their agreements with their respective direct measurements of blood loss.

Results: The haemoglobin mass loss formula met the predefined agreement criterion of ±71 g, with 95% limits of agreement ranging from 0.6 to 44.1 g and a moderate overestimation of 22.4. In comparison to both blood volume loss formulae, the haemoglobin mass loss formula was superior in every agreement parameter evaluated.

Discussion: In this study, the estimation of haemoglobin mass loss was found to be a more accurate method to estimate perioperative blood loss. This estimation method could be a robust research tool, although more studies are needed to establish its reliability.

Figure 1

Histograms of measured haemoglobin mass loss (Hb_MBL) (A) and measured blood volume loss (V_MBL) (B). The amount of each parameter (x-axis) is shown in ranges. Significant blood volume loss was observed: bleeding was = 500 mL in 54% of patients, 1,000 mL in 18% of patients, and 1,500 mL in 7% of patients.

Figure 2

Bland-Altman plots (left) and histograms of differences between estimated and measured blood loss values (right). In the Bland-Altman plot, bias is represented by a solid grey line, 95% limits of agreement are indicated by dashed black lines, and the line of equality is indicated by a dashed grey line. In the histograms of differences between estimated and measured values, the unit conversion (±71 g in 2-B, ±560 mL in (D) and (F) is marked by dashed grey lines, and unit scales were set approximately at 2× the unit conversion (±150 g and ±1,200 mL) to facilitate visual comparison among formulae’ estimations. Although significant bias was observed, the haemoglobin mass loss formula met the agreement criterion of ±71 g (A). The López-Picado’s formula (C) and the empirical volume formula (E) exceeded the ±560 limits of agreement. Using the unit conversion in the histograms of differences, the haemoglobin mass loss formula (B) obtained more accurate estimations than the blood volume loss formulae (D and F). SD: standard deviation.

Figure 3

Bland-Altman plots of percentage differences per formula, for the analysis of the agreements’ consistency. Bias is represented by a solid grey line, 95% limits of agreement are indicated by dashed black lines, and the line of equality is indicated by a dashed grey line. The haemoglobin mass loss formula (A) showed a considerable consistency since estimates became progressively narrower as blood loss increased. In contrast, the López-Picado formula (B) and the empirical volume formula (C) showed no consistency, as dispersion width remained unchanged as blood loss increased. SD: standard deviation.