1Department of Peri-operative Care, Anesthesia and Pain Management, 2Julius Centre for General Practice and Patient Oriented Research, University Medical Centre Utrecht, PO Box 85500, 3508 GA Utrecht, The Netherlands. 3Department of Anesthesiology and Intensive Care Medicine, Twenteborg Hospital, Zilvermeeuw 1, 7609 PP Almelo, The Netherlands. 4Department of Anesthesiology, Isala Clinics, Weezenlanden Hospital, PO Box 10500, 8000 GM Zwolle, The Netherlands*Corresponding author
An abstract about this study was accepted for presentation at the 2000 annual meeting of the American Society of Anesthesiologists (Presentation at San Franscisco, California, USA, October 18, 2000).
Accepted for publication: March 3, 2001
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Abstract |
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Br J Anaesth 2001; 87: 2507
Keywords: blood, transfusion; model, transfusion prediction; anaesthesia, audit
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Introduction |
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We developed and validated a rule based on patient and surgery characteristics, to predict blood transfusion in patients undergoing surgery with intermediate transfusion risk (130%). Subsequently, we evaluated how knowing the preoperative haemoglobin concentration could increase the predictive accuracy of this prediction rule.
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Methods |
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Outcome
The outcome was defined as any allogeneic RBC transfusion (defined as transfusion of one or more units packed cells) on the day of surgery or the first postoperative day. The transfusion decision was made by individual clinicians (anaesthesiologists and surgeons). A rigid protocol was not in use, but in general blood was given when the haemoglobin level was below 10 g dl1 (6.0 mmol litre1).
Potential predictor variables
Age, gender, surgery procedures, whether it was an emergency operation (yes/no), the anaesthetic technique and the preoperative haemoglobin level were evaluated as potential predictors. As 39 different surgical procedures were used, they were allocated into 5 categories based on actual risk (occurrence) of transfusion: Group 1 contained only laparoscopic cholecystectomy (transfusion incidence <5%); Group 2 mastectomy and transurethral resection of tumour (TURT) or prostate (TURP) (transfusion incidence 59%); Group 3 open cholescystectomy, vaginal hysterectomy, Caesarean section, surgery for urine incontinence or vaginal prolapse (1019%); Group 4 non-cardiac thoracic surgery (e.g. lobectomy), vascular (arterial) surgery (e.g. femoro-popliteal bypass), prostate enucleation and endometrial cancer surgery (2029%); Group 5 abdominal and vaginal hysterectomy, hip fracture surgery, revision knee prosthesis, leg amputation, gastro-enterostomy, colon-resection and radical abdominal hysterectomy (30% or more). Anaesthetic technique was defined as a dichotomous variable: a single form of anaesthesia (general, regional or local) compared with a combination anaesthesia (general anaesthesia combined with epidural analgesia). Although in principle a potential predictor, we decided not to include the identity of the surgeon and anaesthesiologist in the model, as they are hard to extrapolate to other hospitals and the aim was to derive an easy and widely applicable prediction rule.
Data collection
The hospital ethics committee approved the study. All data were collected retrospectively from the hospital information system. There were no missing data on any of the predictor or outcome variables, except for the haemoglobin concentration. In 152 patients (10%) it was not measured preoperatively.
Analysis
In the present study, two data sets were randomly selected from all 1482 patients: a derivation set of approximately 75% (1151 patients) and a validation set of approximately 25% (331 patients). SPSS release 9.0 for Windows was used in the analysis (Windows NT 4.0, DELL computer). In the derivation set the association between each predictor and transfusion outcome was quantified using univariable logistic regression modelling. This type of analysis is an alternative to using chi-square tests and gives similar results. In the analysis, surgery was included as four indicator variables (groups 2 to 5) with group 1 as the reference. As the incidence of transfusion in patients aged 18 to 69 was between 10% and 20% in each decade, whereas in patients aged over 70 the incidence increased more rapidly, age was included in the model after dichotomization at 70. After univariate analyses, multivariable logistic regression modelling was applied in order to obtain a prediction model including the independent predictors of transfusion outcome only. This was done by a two-step approach. As age, gender, type of surgery (again included as four indicators), elective surgery and anaesthetic procedure are much easier to obtain, we first evaluated whether these had independent value in the prediction of perioperative transfusion. In this, the interaction between type of surgery and anaesthetic technique was evaluated as well, since both are closely related (regional anaesthesia may reduce blood loss). Subsequently, the added predictive value of the preoperative haemoglobin concentration was evaluated. The full model was reduced by manually (i.e. not automatically) deleting non-significant variables. Predictors with odds ratios that differed significantly from one, defined as odds ratio with P-value <0.10 using log likelihood ratio testing, were considered as independent predictors and retained in the final model. This is commonly done in prognostic research.10
To obtain an easily applicable prediction or scoring rule, the regression coefficients (=ln(OR)) of the predictors in the final model were divided by the smallest coefficient and rounded to the nearest integer. For each subject a score was estimated by assigning points for each variable present and adding the results. The reliability of our prediction rule (goodness of fit) was quantified by using the Hosmer and Lemeshow test. This test is used to compare observed probabilities with predicted probabilities. A high P-value of this test (>0.20) indicates that there is no difference between both probabilities, which means good fit of a model.11 The ability of the model to discriminate between patients with and without transfusion was quantified by using the area under the Receiver Operating Characteristic curve (ROC area).1012 The ROC area can range from 0.5 (no more predictive than a coin flip) to 1.0 (perfect discrimination). A value over 0.7 can be interpreted as reasonable or fair, and over 0.8 as good.13 Differences in ROC area were used to quantify the difference in discriminative ability between full and reduced models taking into account the correlation between the models as they were based on the same cases.14
The performance of the rule was tested in the validation set and the resulting ROC area was compared with the derivation set. A ROC area reflects the overall added value of a model and does not directly indicate its clinical value.15 16 Therefore, in the validation set, we estimated the absolute number of correctly predicted transfused and not transfused patients for the various risk scores of the rule.
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Results |
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This 16% of patients who needed transfusion and who would not have been tested, was only 2% less than using a model with type of surgery as a single predictor. Because age and gender are easily obtainable, we decided to leave them in the model. The sensitivity and specificity of all possible score thresholds can be obtained from the ROC curve in Figure 1.
We wished to reduce the number of unnecessary type and screen procedures (i.e. to obtain a high specificity). We tested whether the preoperative haemoglobin concentration (preopHb), when added to the former prediction model (Table 3), contributed useful information. Adding preopHb to the previous model of Table 3, the ROC area increased from 0.71 to 0.80 (95% CI: 0.740.86) in the validation set. In absolute numbers the percentage of missed transfused patients decreased from 16% to 12%. We reasoned that it would therefore be inefficient to include preopHb in the initial prediction model, as it led to only a small decrease (4%) in missed transfusions at the expense of a haemoglobin measurement in all patients. Nevertheless, using the preopHb additionally after the application of the rule, i.e. only measuring the preoperative haemoglobin level among those patients with score >2, a further reduction in the number of unnecessary type and screen procedures was achievable. Of the 216 patients with score >2 (Table 4), 31 were excluded due to missing values on preopHb, leaving 185. Although we evaluated preopHb as a continuous as well as a dichotomous predictor variable, we decided to use the dichotomised form (at 14 g dl1) to enhance applicability, as there was no difference in predictive accuracy. Table 5 shows the results and can be read in the same way as Table 4. By withholding type and screen procedures in all patients with a preopHb level 14.0 g dl1, a further reduction in type and screen investigations of 24% could be achieved at the expense of another five missed transfused patients. Other haemoglobin thresholds yielded worse results.
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Discussion |
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Some comments are necessary. First, the prediction rules were based on data from one particular hospital. It is known that there are large differences in blood use between hospitals.1723 Although we have tried to show the robustness of the rule by testing it in a second dataset of our hospital, further research has to be done to validate the rule in other hospitals.
Second, the transfusion trigger was a haemoglobin level of 10 g dl1 as was recommended formerly.2426 Currently, fewer patients are transfused as RBC transfusion is now based on the patients risk for developing inadequate tissue oxygenation, which decreases the transfusion trigger to a haemoglobin level between 6 and 10 g dl1.2735 This means that when the proposed prediction rule (including the subsequent preoperative haemoglobin measurement) is applied in current practice, the yearly number of (missed) transfusions will be lower. Rehm et al., for example, found a 26% decline in the number of RBC transfusions when the modern recommendations for transfusions were used.4 This can be inferred from Table 6: the large majority of patients received only 2 units packed cells or less, and would likely not now be transfused. However, although the number of wrongly predicted transfusions will likely be lower, the question about the acceptability of the remaining missed transfusions still exists. As can be seen in Table 6, an emergency transfusion seemed in general not probable (except for two patients who needed re-operation). If a patient is not typed and screened and massive haemorrhage occurs, colloids must be administered and the patient typed and screened. If, however, the patients blood group is not available on time, 0 blood can always be administered even though the presence or absence of irregular erythrocyte antibodies is not yet known.36 Furthermore, the risk of adverse reactions would be low anyway, given the low prevalence of these antibodies in the general population (2.5%).3740 We estimated that in only 0.1% of all transfusions for procedures with intermediate transfusion risk irregular antibodies would be a problem. (The 10 patients in Table 6 who required more than 2 units packed cells count for 3.6% of all transfusions; 2.5%*3.6% = 0.1%.)
Finally, ASA-classification and body mass index are predictors of transfusion.57 9 41 Unfortunately, these variables were not available for most of our patients. This limits the results of the study, although body mass index alone is an objective parameter, but the ASA-classification is proven to be subjective.42 43
The results of this study support previous work. All predictors for transfusion in surgery found in this study (gender, age over 70, surgery procedure and preoperative haemoglobin concentration) were also found by others.59 20 41 4446 However, direct comparison of our rule with other prediction models is difficult as most studies evaluated a particular type of surgery. One study of different types of surgery showed that complexity of surgery procedure, age and preoperative haemoglobin concentration significantly determined the need for perioperative RBC transfusion.6 Most studies included at least one laboratory value in their initial prediction model (preoperative haemoglobin concentration or haematocrit).59 41 Only one study described a rule without a laboratory value, which included similar predictors (e.g. gender) as we found.47 This study only assessed total hip replacement and the outcome was blood loss rather than transfusion. We are the first to construct a prediction rule for surgical transfusion in procedures with intermediate transfusion risk without a laboratory value.
We suggest using our prediction rule at a threshold score of <2 or an estimated risk <10%, as was done by Weber in his model for preoperative prediction of transfusion in cancer surgery.48 Sensitivity and specificity corresponding to this threshold (84% and 40%, respectively) are obtained in a prognostic setting and should not be confused with (usually much higher) estimates obtained in a diagnostic setting. Subsequently, we used the preoperative haemoglobin level at a threshold of 14.0 g dl1. These threshold choices are arbitrary. One could use other thresholds in the scoring rule of Table 4 and the haemoglobin level, though leading to other percentages of misclassifications (Fig. 1).
We assumed that the average direct costs of type and screen are about US$ 80.4850 Using the prediction rule, 35% of all type and screen investigations in intermediate risk surgery procedures could be avoided, giving a reduction in costs of 3 million dollars per 100 000 of these procedures (35 000*$80). When the preoperative haemoglobin concentration is used additionally, a further reduction in costs seems achievable, although the measurement costs of the haemoglobin concentration have to be taken into account. Further cost-effectiveness analyses, including the costs of the missed transfusions, should be done and is topic for further research.
In conclusion, we believe that prediction of blood transfusion in patients having surgery with intermediate transfusion risks is feasible using the rule we have developed together with the preoperative haemoglobin concentration. Using these predictors, the number of preoperative type and screen investigations will be reduced by about 50% leading to a considerable reduction in costs.
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