It is generally accepted that surgical patients who receive blood transfusion intraoperatively are at higher risk of poor clinical outcomes. In one study of liver transplantation, for example, the number of red blood cell (RBC) units transfused intraoperatively was found to be an independent predictor of mortality and graft failure, while preoperative hemoglobin concentration and hematocrit were not.1 Findings such as these demonstrate a need for surgical centers to develop blood conservation programs to reduce unnecessary blood use during surgery.
Intraoperative blood conservation is one objective of patient blood management (PBM), defined as “an evidence-based, multidisciplinary approach to optimizing the care of patients who might need a blood transfusion.”2 PBM programs have become increasingly popular due to growing recognition of the clinical outcomes and costs of blood transfusion. The number of RBC units transfused in the United States, which had gradually increased over a decade to 15 million annually in 2008, has recently begun to decrease (under 14 million in 2011).2 Reasons for this decrease may include economics, as the total cost of an RBC transfusion (incorporating acquisition, delivery, administration, and monitoring) may exceed $1000 per unit,3 and transfusion complications, including potentially fatal reactions such as transfusion-related acute lung injury and transfusion-associated circulatory overload.4
PBM initiatives have been successfully incorporated into surgical programs, reducing the need for intraoperative transfusion.5 These initiatives include identifying patients at risk for transfusion, optimizing red cell mass preoperatively, managing intraoperative coagulopathy and bleeding risk, and using blood salvage techniques.2,5–7 In a multicenter cohort study of 4 hospitals, the development of perioperative PBM initiatives in 4 major areas (education, preoperative optimization of hemoglobin levels, blood-sparing techniques, and standardization of transfusion practices) resulted in a 17% reduction of RBC units transfused per surgical patient without increasing negative patient outcomes such as in-hospital mortality.8
Liver transplantation presents many unique challenges to blood conservation. Patients often have marked hemostatic defects before surgery begins, which become exacerbated during the different phases of transplantation. Thrombocytopenia, platelet dysfunction, coagulation factor deficiency, dysfibrinogenemia, and hyperfibrinolysis are common risks, all of which may exacerbate bleeding and thus necessitate RBC transfusion.5 Several algorithms exist to predict the risk of massive transfusion in these patients using preoperative variables,5,9,10 but many of these factors cannot be corrected before transplantation. There are also no clear guidelines for when intraoperative transfusion is appropriate, though professional society recommendations tend to support restrictive transfusion triggers in most surgical patients (eg, hemoglobin of 7 g/dL or hematocrit 20%).6,7
In the current issue of Anesthesia & Analgesia, Kim et al11 present another obstacle to blood conservation in liver transplantation: error and bias in point-of-care (POC) anemia testing. By retrospectively comparing simultaneous measurements of intraoperative hematocrit by a POC device and a laboratory hematology analyzer in 901 patients, they found that the POC device systematically underestimates hematocrit. Of particular concern, 126 patients had a POC hematocrit <20%, a common threshold for RBC transfusion, but a laboratory hematocrit ≥20%. These patients represented a risk of overtransfusion, with a trend toward receiving more RBC units than those patients for whom both POC and laboratory hematocrit were <20%.
POC testing is a common practice in surgeries where critical therapeutic decision points based on laboratory testing can change rapidly, such as liver transplantation. In addition to hemoglobin and hematocrit monitoring to inform the need for RBC transfusion, POC hemostasis testing may be used to guide the use of plasma, platelets, cryoprecipitate, coagulation factor concentrates, and antifibrinolytic agents. One example of POC hemostasis testing is thromboelastography (TEG), a viscoelastic global assessment of hemostasis. A small randomized, controlled trial of TEG monitoring during liver transplantation demonstrated that the use of TEG was associated with a significant reduction in plasma transfusion and a trend toward reduced transfusion of other blood products, including RBCs12; however, the study included only 28 patients, and a larger trial is needed to confirm the utility of TEG testing in this population.
POC testing may be able to provide accessible results to the surgical team faster than standard laboratory testing, whose turnaround times may be impacted by delays in sample transport, processing, and results reporting. However, the need for rapid results must be balanced against the potentially increased analytic error of POC testing and the possibility of preanalytic error in sample collection and user operation of the device. Furthermore, delays in laboratory testing may be more perceived than actual, and process improvement may reduce many of the actual delays, particularly in transport and reporting.
The methodology of POC hemoglobin and hematocrit testing may differ between devices, which may greatly impact the accuracy of the results.13 Some devices rely on electrical conductivity of RBCs to measure hematocrit, then use an equation to estimate hemoglobin concentration. Other devices directly measure hemoglobin concentration by co-oximetry, then estimate hematocrit. In all cases, the estimating equation (usually a variant of: hematocrit = hemoglobin × 3) assumes that the patient’s sample contains normochromic RBCs without other potential artifacts. These estimates may be incorrect in patients with iron deficiency or hemolysis, providing false triggers upon which the surgical team may act. Other conditions, such as electrolyte disturbances, lipemia, and hypoproteinemia, may cause inaccurate results in the directly measured value.13 Routine laboratory hematology analyzers, on the contrary, directly measure both hemoglobin and RBC indices by separate methods and use several quality rules to identify possible sources of error.14
Recent studies comparing POC to laboratory testing have demonstrated that different POC devices, using different test methodologies, may underestimate or overestimate hemoglobin and hematocrit.15,16 Underestimation may lead to overtransfusion when the POC value is just below a transfusion trigger, while overestimation may lead to missed transfusion when it may have been indicated. A single device may variably show underestimation or overestimation at different hematocrits, further complicating interpretation.15 Kim et al11 found that the risk of a falsely low POC hematocrit was exacerbated by hypoalbuminemia and hypoproteinemia, especially in combination with hyperglycemia. As hypoalbuminemia is common in liver disease, its impact on POC testing during liver transplantation is particularly concerning.
Several strategies may address the potential risks of, or even avoid the need for, POC anemia monitoring for intraoperative transfusion decision-making. POC testing should be compared periodically to laboratory testing in surgical settings in which the POC test is ordinarily used, so that surgical teams can anticipate the possibility of systematic bias and preanalytic error. Surgical centers should evaluate and optimize the delivery of laboratory testing from the point of order to the point of result reporting, to determine if intraoperative POC testing is really necessary; for example, a stat surgical testing pathway may be developed to streamline testing when urgent results are truly needed. Centers should also consider implementing PBM initiatives to reduce the need for intraoperative transfusion.6,7 Finally, more clinical research is needed to establish evidence-based transfusion guidelines in complicated surgeries, such as liver transplantation.
As new POC instruments enter the market, it is critical that they be evaluated by independent studies such as the one performed by Kim et al,11 in order to ensure that they are not only accurate, but also add clinical value for intraoperative monitoring. This can often be difficult in the fast-paced setting of surgery, but it is necessary to confirm whether a new technology aids patient care rather than detracts from it.
Name: Ronald Jackups Jr, MD, PhD.
Contribution: This author reviewed the literature and wrote the manuscript.
This manuscript was handled by: Marisa B. Marques, MD.
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