# Validation of the EuroSCORE Probabilistic Model in Patients Undergoing Coronary Bypass Grafting

^{a}, Antoni Trilla

^{a}, Laia Bruni

^{a}, Raquel González

^{a}, María J Bertrán

^{a}, José Luis Pomar

^{b}, Miguel A Asenjo

^{a}

^{a}Servicio de Medicina Preventiva y Epidemiología, Hospital Clínic, IDIBAPS, Universidad de Barcelona, Barcelona, Spain

^{b}Servicio de Cirugía Cardiaca y Vascular, Hospital Clínic, Universidad de Barcelona, Barcelona, Spain

### Keywords

**Probabilistic model. Quality evaluation. Inhospital mortality. Coronary artery bypass grafting.**

### Abstract

Introduction and objectives. EuroSCORE utilizes a probabilistic model for predicting the risk of in-hospital mortality in patients undergoing cardiac surgery. It is a useful instrument for evaluating quality of care. The model has two variants: the logistic EuroSCORE and the additive EuroSCORE. The aim of this study was to validate the EuroSCORE model in patients undergoing surgery at Hospital Clínic in Barcelona, Spain, and to compare the results obtained with the two variants. Methods. The study included all patients who received a coronary artery bypass graft (CABG) at Hospital Clínic in Barcelona in two consecutive years. The model's validity was assessed on the basis of its calibration (using the Hosmer-Lemeshow test) and its discrimination (using the receiver operating characteristic [ROC] curve). The two models were compared by carrying out a descriptive analysis of mortality for the whole group and for different risk groups, and by determining the models¿ discriminative power. Results. A total of 498 patients underwent CABG surgery and were included in the study. The Hosmer-Lemeshow test showed that the model¿s calibration was satisfactory (P=.32) and the area under the ROC curve was 0.83. The observed in-hospital mortality rate was 5.8%. The predicted rate was 4.2% with the logistic EuroSCORE and 3.9% with the additive EuroSCORE. Large differences were observed in high-risk patients. In these patients, the mortality predicted by the logistic variant was closer to the actual mortality. Conclusions. EuroSCORE¿s validity was found to be satisfactory and the model can be used to evaluate quality of care. In high-risk patients, mortality estimated using the logistic model was closer to the actual mortality.### Article

**
INTRODUCTION**

Health-care
providers need to be able to reliably assess their activities in
terms of outcomes, quality, and cost-effectiveness. This is, in
part, due to the constant rise in health care costs and the fact
that resources are limited, though other drivers include the
increased demand for health services and the need to compare
clinical outcomes between centers. As all institutions require
information on quality of care they need to be able to summarize
their activities in terms of outcomes adjusted by the center's
specific characteristics.^{1}

In the area of
heart surgery, both in Europe^{2} and the United
States,^{3} the periodic publication of outcomes reports is
now widespread and adequately regulated.^{4,5} These
reports incorporate information from mathematical models which are
used to predict the likelihood of a certain event, such as death,
occurring in a given individual based on a group of risk factors
attributed to that particular patient.^{6}

The European
System for Cardiac Operative Risk Evaluation (EuroSCORE) is a
logistic model which is used to predict hospital mortality in
patients undergoing a cardiac intervention. Using 18 risk variables
and a beta coefficient associated with each variable (Table 1), the
model provides the likelihood of death for any individual. The
model was created and initially validated in a cross-sectional
study^{7,8} of 19 030 European patients in 1999. Since
then, it has become the most widely used model worldwide in this
type of patient.

A much simpler
variant of the logistic model is the additive EuroSCORE, which
assigns a weight to each risk factor presented by the patient. The
sum of the weights provides the likelihood of dying for that
patient. The widespread and uniform use of a single probabilistic
model allows for internal and external comparisons over time and
can help to minimize risk adverse behavior which might be fomented
if comparisons are made using unadjusted
outcomes.^{9,10}

The primary
objective of the present study was to validate this predictive
model of mortality in a large teaching hospital in Barcelona,
Spain. The model was assessed in terms of fit and discriminatory
capacity.^{11}A second study objective was to compare the
additive and logistic versions of the model and to determine which
was the most appropriate for use in different groups of patients
defined by their level of risk. The additive version has been the
most widely used of the model variants because, although it is less
precise, it is much easier to calculate and it can be calculated at
the bedside. Nevertheless, the logistic equation has been shown to
better predict mortality, particularly in high risk patients, and
is recommended for use in those
patients.^{12-14}

**
METHODS**

**
Patients**

This was a validation study performed in the Hospital Clínic of Barcelona (HCB) using a retrospective design. The hospital is a 720-740 bed center which deals with 40 000 admissions annually. Patients with cardiovascular disease are attended in the Clinical Institute for Diseases of the Thorax which includes, among others, the Cardiovascular Surgery and Cardiology services. The study was approved by the Institutional Review Board of the HCB.

The center's
computerized database (SAP^{®}) was used to obtain data
on all patients who underwent procedures defined in the
CIE-9-CM^{15} and who were assigned codes 36.10 to 36.17
and 36.19. The SAP is used for both clinical and administrative
purposes and has high reliability. All discharges are coded. The
study population consisted of patients derived for coronary surgery
and who underwent surgery between July 1, 2004 and July 1,
2006.

**
Variables and Statistical Analysis**

The 18 variables included in the EuroSCORE predictive model were identified for all patients included in the study (Table 1) together with administrative variables (date of admission and discharge) and data on deaths occurring while patients were hospitalized in relation to the intervention. These data were obtained from computerized clinical records which included discharge history as well as surgery, pre-anesthesia, and laboratory reports. If no value was recorded for a specific risk factors, it was assumed that the risk factor was absent.

Both the logistic
and additive versions of the Euro-SCORE^{12} were used to
predict mortality. In the case of the additive EuroSCORE, the
probability of dying was calculated by summing the relative weights
for each risk factor for all individuals.

In order to calculate predicted mortality using the logistic model, the following equation was used

where
b
_{0} takes a value of
-4.789594 (logistic regression constant) and
b_{
i}
is the regression coefficient
for the variable
c_{
i}
in
Table 1. For the age variable, in the logistic method
b
was multiplied by the number of
years that the patient exceeded 60 years of age. In the additive
method, a weight of 1 was assigned for every 5 years (or part of 5
years) over 60.

The validity of
the logistic regression model was analyzed by examining its
goodness of fit and discriminatory capacity. Goodness of fit was
assessed using the Hosmer-Lemeshow test^{16-19} which
estimates a C statistic from the difference between observed and
expected values for mortality in different risk groups. The lower
the C statistic the better the model's fit. A value of *
P*
>.05 indicates that the model
fits the data well and that it therefore accurately predicts
mortality. The test is most frequently used to validate recently
created models but it is equally useful in validating an existing
model which has been applied in a new set of data, as in the
present study.

The model's
ability to discriminate is assessed in terms of its capacity to
distinguish between patients who died during hospitalization from
those who did not. Discriminatory capacity was analyzed by
calculating the area under the ROC curve. A value of 0.5 indicates
that the model is equivalent to pure chance and a value of 1
indicates perfect discrimination.^{17}

The logistic and
additive models were compared by calculating the mortality
predicted by each in both the overall sample and in 2 sub-groups
defined by level of risk. The high and low risk groups were defined
using a cut-off point on the additive EuroSCORE of 6
points,^{6} a cut point which had previously been used for
this purpose.^{8,12} ROC curves were calculated for both
models.

The validation
analysis was carried out using the STATA^{®} v.8
statistical package and the comparative analysis using
SPSS^{®} v.12.0.

**
RESULTS**

A total of 498 patients underwent heart surgery in HCB between July 1, 2004 and June 30, 2006. The distribution of EuroSCORE risk variables in these patients is summarized in Table 2.

For the logistic
model, a C statistic of 11.51 (*
P*
=.32) was obtained on the
Hosmer-Lemeshow test and the area under the ROC curve was 0.83
(Figure 1).

**
Figure
1.**
ROC curve for EuroSCORE logistic model.

Among the patients who underwent surgery during the study period, there were 29 hospital deaths, giving an overall mortality rate of 5.8%. Total predicted mortality was 3.9% using the additive model and 4.2% using the logistic model (Table 3).

In the low risk group (n=412), observed mortality was very similar to that predicted by both models; however, in the high risk group (n=86) the values predicted by the logistic model were closer to observed values (Table 3). Both models showed good discriminatory power, with an area under the ROC curve of 0.84 for the additive model and 0.83 for the logistic model (Figures 1 and 2).

**
Figure
2.**
ROC curve for EuroSCORE additive model.

**
DISCUSSION**

In order to
evaluate the quality of health care services and to be able to
adequately inform patients on the likely outcomes of the health
care process, crude values for overall observed or expected
outcomes are often not sufficient. Prognostic models which take
into account patients' specific characteristics and which provide
risk-adjusted outcomes for interventions are required and more
useful.^{6}

There are many
risk-adjusted models available to predict mortality in cardiac
surgery interventions, though in recent years the
EuroSCORE^{
8}
has become one of the most widely used in western
countries.

The HCB is a reference center for this type of heart surgery and performs a large number of coronary interventions. In this type of center, reliable and comparable data are required in order to assess the quality of care.

Before using a
probabilistic model in a context other than that for which it was
created,^{20} the model should be validated to ensure that
it does not generate erroneous probabilities. The aim of the
present study was to validate^{
21}
the EuroSCORE in the
HCB.

The C statistic
obtained with the Hosmer-Lemeshow was *
P*
=.32, which indicates
satisfactory model fit for patients undergoing surgery in our
center. The model's discriminatory power was also adequate, as
indicated by an area under the ROC curve of 0.83. Given these
results, we can conclude that the EuroSCORE model has been
validated for use in this center and that it has proven to be a
reliable instrument. This signifies that the model's predictions of
the probability of dying are valid and appropriately risk-adjusted
for surgery patients in HCB. It should be pointed out that
mortality during hospitalization is a very favorable measure of
mortality as it does not incorporate mortality after discharge.
Nevertheless, we believe that intervention-related mortality after
discharge was practically null in this series.

^{22}to which Spain contributed 2422 patients. In the earlier study, the Hosmer-Lemeshow C statistic was

*=.38 and the area under the ROC curve was 0.87. Nevertheless, the earlier validation in Spain took place in very diverse settings and conditions, which meant there was a need for further validation in specific contexts before the model could be used with confidence.*

*P*
The additive, or
standard, model has been the most widely used because it is easy to
use. It is a simplified version of the logistic model and the
weights it uses are derived from that model. A previous
study^{12} indicated that the additive model tended to
underestimate the probability of death in high risk
patients.

In the comparison
of the 2 models, only low and high risk groups were studied because
of the relatively low mortality in the study population. Creating a
larger number of risk groups would have led to very broad
confidence intervals for the predicted mortality rates and would
have hindered comparisons. The cut point for defining the 2 groups
was a EuroSCORE value on the additive model of >6. In this
study, we found that the logistic model more accurately predicted
the probability of death in the high risk group, a result in line
with previous studies which have compared the 2
models.^{12}

A study in a larger sample would generate a larger number of events (deaths) and would provide more solid results as the Hosmer-Lemeshow test is based on contrasting predicted events with observed events.

A direct comparison of real and expected events was not carried out as they are two distinct variables which provide information about events. True mortality describes the event death, whether observed or otherwise, for each patient (dichotomous variable). Predicted mortality, on the other hand, indicates the likelihood of dying for each patient based on specific characteristics included in the model (quantitative continuous variable). The analysis of the relationship between these 2 variables is precisely what constitutes the validation of the model. The validated model is useful because it allows us to perform risk assessments for patients which can then be compared with observed outcomes, while taking into account the level of risk.

The publication of
the outcomes of health care has been a reality in the United States
and the United Kingdom for over 15 years. The impact of such
publications has been extensively analyzed, with the results
showing a clear improvement in the quality of health
care.^{23} The instrument validated in the present study
could be useful in providing systematic information on the outcome
of interventions in centers providing the relevant services. The
information could be published and made available both to citizens
and the purchasers of health care.

**
CONCLUSIONS**

The results of this study allow us to conclude that the EuroSCORE is a useful probabilistic model in this public, teaching hospital. It can be used to estimate the probability of death in patients scheduled for heart surgery and to assess the outcomes of health care. The logistic model is the most reliable of the 2 versions, particularly in high risk patients.

ABBREVIATIONS

HCB: Hospital Clínic of Barcelona

EU-A: additive EuroSCORE

EU-L: logistic EuroSCORE

EuroSCORE: European System for Cardiac Operative Risk
Evaluation

ROC: receiver operating characteristic

**
SEE EDITORIAL ON
PAGES 567-71**

Correspondence:

Dr. A. Trilla.

Servicio de Medicina Preventiva y Epidemiología. Hospital Clínic. Villarroel, 170. 08036 Barcelona. España.

E-mail: atrilla@clinic.ub.es

Received
August 11, 2007.

Accepted for publication December 19, 2007.

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