Shed Mediastinal Blood Transfusion After Cardiac Operations: A Cost-Effectiveness Analysis

Shed Mediastinal Blood Transfusion After Cardiac Operations: A Cost-Effectiveness Analysis Meredith L. Kilgore, MSPH, and Albert D. Pacifico, MD Department of Pathology and Division of Cardiovascular and Thoracic Surgery, University of Alabama at Birmingham, Birmingham, Alabama

Background. Cardiac surgical patients consume a significant fraction of the annual volume of allogeneic blood transfused. Scavenged autologous blood may serve as a cost-effective means of conserving donated blood and avoiding transfusion-related complications. Methods. This study examines 834 patients after cardiac operations at the University of Alabama Hospital. Data were collected on patients receiving unwashed, filtered, autologous transfusions from shed mediastinal drainage and those receiving allogeneic transfusions. The data were incorporated into clinical decision models; confidence intervals for parameters were estimated by

bootstrapping sample statistics. Costs were estimated for transfusing both allogeneic and autologous blood. Results. The study found a 54% reduction in transfusion risk or a mean reduction of 1.41 allogeneic units per case (95% confidence interval, 1.04 to 1.79 units). The process saved between $49 and $62 per case. Conclusions. The use of autologous blood has the potential to significantly reduce the costs and risks associated with transfusing allogeneic blood after cardiac operations. (Ann Thorac Surg 1998;65:1248 –54) © 1998 by The Society of Thoracic Surgeons

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Material and Methods

etween 12 and 14 million units of allogeneic blood products are transfused annually in the United States [1, 2]. Cardiac surgical patients account for roughly 10% of the 3.2 million annual recipients [2]. Transfusions confer life-saving benefits, but impose serious risks, including transfusion reactions, blood-borne infections, and immunosuppression. A variety of approaches can minimize exposure to allogeneic blood: improved hemostasis; autologous blood obtained before or during the operation; and the transfusion of blood scavenged from wounds perioperatively. This study analyzes the cost-effectiveness of transfusing unwashed, filtered mediastinal chest tube drainage after cardiac operation for coronary artery bypass grafting, septal defect closure, or valve repair. Throughout, the terms autologous blood and autotransfusion refer to postoperative reinfusion of a patient’s own blood scavenged from chest drainage. The perspective taken is that of the payers for and providers of cardiac surgical services. Treatment costs for adverse outcomes are included in the analysis, and consideration of the costs of non– quality-adjusted lifeyears saved is taken in the conclusion. This perspective takes advantage of reliable information on costs, but by omitting costs incorporated from a societal perspective [3], introduces a bias against postoperative autotransfusion. We therefore tend to understate its potential costeffectiveness. Accepted for publication Dec 5, 1997. Address reprint requests to Mr Kilgore, Department of Pathology, The University of Alabama at Birmingham, P230 West Pavilion, 619 South 19th St, Birmingham, AL 35233-7331 (e-mail: [email protected]).

© 1998 by The Society of Thoracic Surgeons Published by Elsevier Science Inc

Data were obtained from patients undergoing cardiac operations at the University of Alabama Hospital at the University of Alabama at Birmingham. Table 1 provides demographics describing the sample. The patients included all adults undergoing coronary artery bypass grafting, valve replacement or repair, and cardiac transplantation during the period from October 1996 through June 1997. Five attending surgeons perform cardiac operations at UAB, which maintains an accredited cardiovascular residency program. After operations, patients arrive directly from the operating room to the cardiothoracic intensive care unit (CICU). The CICU uses a fluid-management system (FMS 1000; Advanced Data Systems, Birmingham, AL) to monitor mediastinal drainage and urine output and to reinfuse autologous blood as indicated. Autologous blood is reinfused according to a fixed protocol. Once a patient has shed 150 mL of mediastinal drainage, autotransfusion is initiated. Autotransfusion is suspended if central venous or pulmonary artery diastolic pressures show evidence of fluid overload. Data from the FMS 1000 is transmitted to a CareVue 9000 clinical information system (Hewlett-Packard, Baltimore, MD); rates of chest tube drainage, urine output, and autotransfusion are incorporated into an electronic medical record. Data on hemoglobin levels, blood loss, blood products infused, and patient demographics were extracted from the CareVue 9000 system using its database export utility. These data were imported into a SAS (SAS Institute, Cary, NC) database for statistical analysis. Simple statistics were obtained, a x2 analysis was run, and ordinary 0003-4975/98/$19.00 PII S0003-4975(98)00140-4

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Table 1. Demographic Statisticsa

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Table 2. Bootstrap Statistics

Variable

No.

Mean

Standard Deviation

Age (y) Height (cm) Weight (kg) Body surface area (m2) Length of stay (h)

615 604 615 604 512

61.9 170.9 80.9 1.978 32.64

12.7 12.8 17.0 0.249 18.7

a

There were 200 female patients (32.6%) and 413 male patients (67.4%). There were 221 patients for whom data were missing.

least squares and logistic regression models were tested. Because of the nonnormal error distributions and collinearity among variables, regression models are inappropriate. Instead, tiered x2 analyses (described below) were used to generate probabilities for clinical decision analysis and cost-effectiveness models. Decision trees were constructed using Decision Analysis (TreeAge Software, Inc, Williamstown, MA). Bootstrapping the sample statistics [4] provides empirical estimations of means (and 95% confidence intervals). This technique has been demonstrated useful when the assumptions required for t tests, analysis of variance, and ordinary least squares models cannot be met. Bootstrapping involves randomly sampling the data, with replacement, to obtain a new sample of equal size. The process is iterated many times (1,000 iterations were used in this study). Resampled means are sorted in ascending order, and uncorrected 95% confidence intervals estimated by counting up from the lowest 2.5% and down from the highest 2.5% (in this instance the 26th and 975th) of the resampled means. The confidence intervals thus obtained are biased estimates, but bias-correction can be obtained from the rank of the bootstrapped mean (S 5 mi/n where n is the number of iterated means) within the ordered sequence of resampled means, then using this as the basis for determining the upper and lower confidence limits [5]. The value of bootstrapping is that the only assumption that needs to be met is that the sample distribution is similar to that of the population being sampled. This amounts to a nonparametric approach to estimating population parameters. Table 2 details sample and bootstrapped means and 95% confidence intervals derived from our data set. We use units of blood products because the risks of bloodborne infections and transfusion reactions derive from the units infused. The risks of pooled platelet transfusion are not considered here, on the assumption that autotransfusion will not return viable platelets to a patient nor reduce the need for platelet transfusions. Tiered x2 analysis was constructed as follows. Cross-tabulations estimate the probabilities of receiving a first allogeneic unit, then iterated cross-tabulations were constructed to yield the probability of receiving another unit for each one received thereafter. Sample size precludes carrying the analysis further than five or more units transfused, as expected cell values lower than five cannot be used to calculate reliable x2 statistics. The mean number of units

Variable

Bootstrap Sample Bootstrap 95% Confidence Mean Mean Interval

Allogeneic units Autotransfusion (mL) Chest tube drainage (mL) Probability of receiving allogeneic blood “Risk_1” probability of Hgb #7.0 g/dL or CTD $250 mL “Risk_2” probability of Hgb #7.0 g/dL or CTD $500 mL

1.7 296 641 0.305

1.7 295 641 0.304

1.4 –2.0 269 –322 602– 683 0.276 – 0.337

0.819

0.807

0.779 – 0.836

0.661

0.661

0.628 – 0.697

CTD 5 chest tube drainage;

Hgb 5 hemoglobin.

was 11 for patients who received at least 5 units postoperatively. Figure 1 shows the cross-tabulation of observations for patients who did and did not receive autotransfusion (rows) with those who did and did not receive banked blood (columns). Roughly 39% of these patients received no allogeneic blood and no autotransfusion. Of those patients who received no autotransfusion 24.4% did receive allogeneic blood; of those who did receive autotransfusion, 36.3% received at least one unit of donor blood. The x2 statistics (Table 3) indicate that the distribution is unlikely to have occurred by chance (p , 0.001). (The full tier of cross-tabulations used to complete the decision tree shown in Figure 3 can be made available upon request.) Two decision trees were constructed [6]. Each tree begins with the decision of whether or not to make

Fig 1. Cross-tabulations for autologous by allogeneic blood transfusion (see Table 3 for statistics).

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Table 3. Statistics for 2 3 2 Table Statistic

x Continuity adjusted x2 Mantel-Haenszel x2 2

DF

Value

p Value

1 1 1

22.071 21.370 22.044

0.001 0.001 0.001

DF 5 degrees of freedom.

autotransfusion available. The branches of the tree delimit the consequences arising from the decision, with their associated probabilities. The terminal branches of the decision tree are assigned “payoffs,” a term of art which means the quantified outcomes (eg, morbidity, mortality, health, costs) that arise probabilistically from a specified set of conditions. The first tree (Fig 2) depicts the effect of making autotransfusion available on the simple probability of receiving allogeneic blood. The second tree (Fig 3) goes on to examine the number of units received; at each branch the patient receives a single additional unit or does not. The terminal branches are assigned payoffs of the number of units received (with those patients going on to receive 5 allogeneic units being assessed the mean payoff of 11 units for that subgroup). Patient characteristics were examined to determine whether those receiving autotransfusion differed significantly from those who did not. A x2 analysis found no

Fig 2. Simple autotransfusion decision tree. (ATN 5 Autotransfusion.)

Ann Thorac Surg 1998;65:1248 –54

significant differences among surgeons (p 5 0.79). General linear models found no significant differences according to body surface area (p 5 0.91) or length of stay (p 5 0.26). Older patients tended to receive more autotransfusion (p 5 0.01), as did men (p 5 0.03). Probabilities associated with the decision not to provide autotransfusion to this patient sample must be inferred. All of the patients in this study had autotransfusion available to them. Those that did not receive it either had no significant bleeding, or began bleeding after too long a time interval (4 hours, by protocol) to allow autotransfusion to be safely initiated. Our estimate for this set of probabilities relies on hemoglobin levels and rates of bleeding. Patients were assumed to require blood products if the nadir of the postoperative hemoglobin level reached 7 or 8 g/dL, and if chest tube drainage reached alternatively 250 or 500 mL postoperatively. Sensitivity analysis was done to test the effects of varying the parameters, and bootstrapping was used to estimate confidence intervals. Once the initial odds of receiving the first unit of banked blood were inferred, the probabilities of receiving each additional unit were estimated by those patients receiving no autotransfusion but who did require banked blood products. This design eliminates any potential effects of autotransfusion on rates of bleeding. A second set of payoffs includes the costs of providing autotransfusion, the costs of transfusing banked blood, and the risks and costs of adverse sequelae. The major costs for the autotransfusion system must be borne by all patients (or payers) regardless of its utility in any given case. No reliable protocol has been developed to predict which patients will require postoperative transfusions [7]. Hence, the cost decision must be made for the population of patients being served by the facility. The costs of providing autotransfusion include the costs of equipment and disposables that would not be otherwise incurred. Capital costs for this analysis include only that equipment required to deliver autotransfusion. The FMS 2000 system primarily functions as a monitoring system, providing real-time data on a patient’s fluid balance status. The costs assigned for autotransfusion were limited to the computer-controlled infusion pump integrated into the system. The disposable costs include the use of a cardiotomy reservoir with a built-in 20-mm filter and autotransfusion kits used only when autotransfusion is delivered. The volume infused, however, does not affect the costs of providing autotransfusion. They are all-or-nothing costs incurred at the decision points for making the service available ($88 per case), and of delivering autotransfusion in a specific case given that availability ($16.67 for the requisite intravenous tubing and cassette). Tables 4 and 5 outline the cost estimates for a unit of allogeneic blood. The tables are adapted from a study by Lubarsky and colleagues [8], at Duke University, updated with cost and risk data from more recent articles [9 –12]. We estimate marginal costs for allogeneic units. The costs of initial typing and screening, fixed costs, and the costs of blood bank administration are excluded, as making

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Fig 3. Full decision tree. (ATN 5 autotransfusion; EV 5 expected value.)

autologous blood transfusion available will have negligible impact on the transfusion service. This method yields an estimate of $107.70 (range $105.77 to $113.27) for the cost of one unit of transfused allogeneic blood. The variation results from assumptions as to the costs of Table 4. Cost Summary Item Unit PRBCs Laboratory handling costs Administration Handling and wasting unused units Cost of treating complications Total costs

Cost/Unit $76.50 $11.79b

N/A N/A

$5.27b $7.67b

N/A N/A

$6.47c

$4.54 –$12.06

$107.70

a UAB Hospital Blood Bank Quote, January 1997. c See Table 5. [8].

PRBCs 5 packed red blood cells.

Range

a

$105.77–$113.27 b

Lubarsky et al

treating adverse sequelae (outlined in Table 5). Risks of blood-borne infection (eg, hepatitis B, C, or non-A/nonB/non-C; human immunodeficiency virus) sum to roughly 1 in 36,000 per allogeneic unit. Treatment costs per unit sum to $3.15 ($1.22 to $8.74) for these diseases. The risk of a clinically significant transfusion reaction is 1 in 264 (1 in 3,700 for the most severe reactions). Riskadjusted treatment costs for transfusion reactions sum to $3.32 per unit of allogeneic blood transfused. Our cost estimates are consistent with those reported by Roberts and associates [13] of $100.89 to $107.26 per unit, but lower than those reported by Etchason and coworkers [14] of $149.80 per unit. Goodnough and colleagues [2] found production costs of between $95 and $120 before consideration of the costs of adverse sequelae. These costs were computed in 1993, and include overhead costs that we omit for the reasons stated. The costs of allogeneic blood will likely be site-specific and difficult to generalize. Results in a clinical decision model are obtained by “rolling back” the tree. The payoffs at the terminal nodes

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Table 5. Costs of Adverse Sequelae

Type of Complication Infections HIV Hepatitis B Hepatitis C Transfusion reactionsb Anaphylactic Hemolytic Febrile Other

Range

Cost Per Unit

Unit Cost Range

$119,274c $117,000d $117,000d

$105,760 –$132,789 $115,000 –$119,000 $115,000 –$119,000

$0.24 $1.87 $1.04

$0.04 –$0.65 $0.78 –$3.84 $0.40 –$4.25

$5,125 $5,125 $51 $1,136

N/A N/A N/A N/A

$0.56 $0.82 $0.10 $1.84

N/A N/A N/A N/A

$6.47

$4.54 –$12.06

Estimated Risk/Unit

Range

Treatment Costs

1/493,000a 1/63,000a 1/103,000a

1/(202,000 to 2,778,000) 1/(31,000 to 147,000) 1/(28,000 to 288,000)

0.00011 0.00016 0.00200 0.00162

N/A N/A N/A N/A

Total a

Schreiber et al [9].

b

Lubarsky et al [8].

HIV 5 human immunodeficiency virus;

c

Hellinger [11].

d

Etchason et al [14].

N/A 5 not applicable.

(the triangular end points of the tree) multiplied by the probabilities assigned to the branches of the most proximate chance nodes (circular focal points within the tree) and these products are summed to yield an expected value. This value constitutes the payoff for that chance node, and the process is carried on until a decision point (square node) is reached. The optimal course will be reflected by the best (highest good, lowest bad) expected value.

Results The first decision tree was rolled back to compute expected values for the number of allogeneic blood units transfused. Provision of postoperative autotransfusion reduces the need for allogeneic transfusions by roughly 54%. The full decision tree was rolled back for costs and for units of allogeneic blood transfused. Using our most conservative estimate of transfusion risk (nadir hemoglobin #7.0 g/dL or chest drainage $500 mL, a 66% probability), autotransfusion after operation yields savings of $55 per case when the costs of allogeneic transfusion were $107.70 per unit. The expected value when autotransfusion is available came to $273 versus $322 if only banked blood products were used. Autotransfusion saves an average of 1.4 units of allogeneic blood products per case (expected value of 1.7 units with autotransfusion versus 3.1 units without). A sensitivity analysis was performed in which the probability of receiving allogeneic blood in the absence of available autotransfusion is varied from 0% to 75%. When the risk exceeds 55.2%, then the costs of providing autotransfusion to patients after a cardiac operation are less than the costs associated with using only banked blood products. Bootstrapping yielded a bias-corrected 95% confidence interval for the probability of receiving a first unit of allogeneic blood products of from 0.628 to 0.697 for our transfusion criteria (nadir hemoglobin #7 g/dL or chest drainage #500 mL), and from 0.779 to 0.836 if a less stringent criterion (chest drainage #250 mL) was applied. This implies that autotransfusion is likely to be

accompanied by benefits on the order of those suggested by the decision analysis model. Autotransfusion can lower the cost of transfusion services after cardiac operations. The cost analysis tended to be insensitive to changes in the cost of a unit of banked blood over the narrow range chosen for our cost estimate. Cost differences ranged from $49 to $62 per case if unit costs for banked blood were varied from $105.77 to $113.27. If, however, the costs of banked blood reached the $148 suggested by Etchason and coworkers, then providing autotransfusion postoperatively saved an average of $112.00 per case. A further sensitivity analysis was performed on the potential effect of capturing social costs [3]. The total costs associated with the most serious adverse transfusion sequelae (human immunodeficiency virus, hepatitis B virus, or hepatitis C virus infection, and anaphylaxis of hemolytic transfusion reactions) were arbitrarily varied from those used in the computations for Table 5. These costs were set at $150,000, $250,000, and $500,000; this translates into costs of $140, $170, and $244 per allogeneic unit. At $150,000 per severe complication, autotransfusion provides savings of $100 per patient. At $250,000 per complication, the savings is $142 per patient, and at $500,000 per complication, autotransfusion produces a savings of $246 per patient.

Comment This study will not be the last word on postoperative transfusion of shed mediastinal blood. Some studies have reached similar, others contrary findings [7, 15, 16]. Furthermore, a prospectively randomized study would yield more convincing results. There are substantial challenges to any study of autotransfusion. The distribution of bleeding among postoperative patients is subject to high variance and is extremely skewed. These factors invalidate standard statistical tests of significance, such as the Student’s t test or other linear models. Additionally, factors that create the need for blood transfusion (bleeding, low hematocrit) correlate with autotransfusion

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to the same degree, creating a multicollinearity problem. Finally, there is the problem of statistical power. The principal putative beneficiaries of autotransfusion are patients with significant bleeding. This means that studies must be very large to detect significant differences in allogeneic blood use. It is hoped that the results from this study, and some of the analytic techniques, can be used to justify the expense of a large enough randomized study of this question. As noted above, one critical factor in evaluating the cost-effectiveness of postoperative autologous transfusion available is the probability of patients requiring allogeneic blood if autotransfusion is not made available. It is unreasonable to assume that each equivalent unit of autologous blood avoids the transfusion of one unit of allogeneic blood products. Clinicians may be willing to return autologous blood when they would consider the transfusion-related risks grounds for restraint in their use of banked blood. Additionally, there is wide variation in transfusion practice. Average rates of transfusion after cardiac operations reported in the literature range from 52.3% up to 68% [17, 18]. In a study from the United Kingdom, a randomized control trial of the transfusion of unwashed, filtered blood after total knee replacement found a 68% reduction in allogeneic transfusions [19]. Studies have also reached different conclusions as to whether autologous blood is more or less costly per unit than allogeneic products [2, 13, 14]. Care should be taken in deciding which costs to impute to banked blood products. The availability of postoperative autologous blood transfusion will not obviate the need for transfusion services. It is therefore necessary to compare costs at the margins. Including overhead costs in such an analysis will have the perverse effect of autotransfusion creating higher costs for allogeneic blood products (fixed costs hold constant as total volume declines). A failure to focus on the true opportunity costs would imply savings that would not materialize once a decision was taken. The costs of treatment associated with blood-borne infections and with transfusion reactions remain problematic. We have not included costs arising from prolongation of hospital stays because of infections related to transfusion-induced immunosuppression. Although there is evidence to support the existence of such effects, we found no adequate information on the associated costs [20]. Because this study adopts the viewpoint of health service providers, costs resulting from illness or death are limited to treatment costs. From a societal perspective, a broader scope of costs would have to be considered. One approach to estimating the cost-of-illness, the “human capital” method, would include foregone productivity and the costs of unreimbursed care provided by family or significant others. An even broader approach would capture the mortality and morbidity costs associated with pain and suffering and the present value of future earnings [3]. Finally, there is the question of fitting this study into the paradigm of cost-effectiveness analysis. Using data from 1990 [21], transfusing 2 allogeneic units to a 50-yearold patient resulted in an average of 18 days of life lost

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(from non-A, non-B hepatitis; hepatitis B; transfusion reactions; and human immunodeficiency virus infection). Because autotransfusion results in lower postoperative costs, the opportunity cost is negative. When a cost to effect ratio is negative because of a negative value in the numerator, the option being evaluated strongly dominates the alternative [22]. Because costs that arise from the patient or societal perspective are omitted or unavailable, our study tends to understate the potential savings. The case can be summarized simply. Using conservative estimates, postoperative autologous transfusion reduces a patient’s risk of receiving allogeneic blood after a cardiac operation by 54%, with net savings in costs. We thank Dr Stephen T. Mennemeyer, Dr Eli I. Capilouto, and Dr Michael A. Morrisey of the Department of Health Care Organization and Policy at the University of Alabama at Birmingham School of Public Health, Dr John A. Smith, Professor of Pathology and Director of Laboratory Medicine UAB, Dr Richard C. Friedberg, Director of Transfusion Medicine at the Veterans Administration Hospital, Birmingham, and CoDirector of Transfusion Medicine, UAB Department of Pathology, and Dr Glen L. Hortin, Head of the Clinical Chemistry Section in the UAB Department of Pathology; all contributed constructive criticism to this study. We are also grateful to anonymous reviewers for The Annals for their comments and suggestions. Finally, we extend our thanks to the nursing staff of the Cardiothoracic Intensive Care Unit at the UAB Hospital for their cooperation and support.

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13. Roberts WA, Kirkley SA, Newby M. A cost comparison of allogeneic and preoperatively or intraoperatively donated autologous blood. Anesth Analg 1996;83:129–33. 14. Etchason J, Petz L, Keeler E, et al. The cost effectiveness of preoperative autologous blood donations. N Engl J Med 1995;332:719–24. 15. Page R, Russell GN, Fox MA, Fabri BM, Lewis I, Williets T. Hard-shell cardiotomy reservoir for reinfusion of shed mediastinal blood. Ann Thorac Surg 1989;48:514–7. 16. Roberts SR, Early GL, Brown B, Hannah H, McDonald HL. Autotransfusion of unwashed mediastinal blood fails to decrease banked blood requirements in patients undergoing aortocoronary bypass surgery. Am J Surg 1991;162:477– 80. 17. Goodnough LT, Johnston MF, Toy P. The variability of transfusion practice in coronary artery bypass surgery. JAMA 1991;265:86–90.

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18. Chiavetta JA, Freedman HJ, Axcell TJ, Wall AJ, van Rooy SC. A survey of red cell use in 45 hospitals in central Ontario, Canada. Transfusion 1996;36:699 –706. 19. Henderson MS, Newman JH. Reinfusion of unwashed salvaged postoperative blood following knee replacement [Abstract]. Transfusion 1996;36:661. 20. Murphy PJ, Blumberg N, et al. Allogeneic blood transfusion as a risk factor for postoperative infection after coronary artery bypass graft operations. J Thorac Cardiovasc Surg 1992;104:1092–9. 21. Carson JL, Russell LB, Taragin MI, Sonnenberg FA, Duff AE, Bauer S. The risks of blood transfusion: the relative incidence of acquired immunodeficiency syndrome and non-A, non-B hepatitis. Am J Med 1992;92:45–51. 22. Stokey E, Zeckhauser R. A primer for policy analysis. New York: WW Norton & Co, 1978:126.

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