Cardiovascular Disease Risk Estimation: Why, When and How?
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Cardiovascular disease risk estimation: Why, when and how? Hermione Price Article points Risk assessment is fundamental to preventative medicine. Without 1. Estimating cardiovascular disease (CVD) risk it, people may not be identified as being at high risk of a disease, allows the identification and the chance to prevent that disease occurring may be lost. of individuals without There are a variety of cardiovascular disease risk calculators major risk factors who are actually at high risk. available. Each has been developed from a different population Identifying such people and each has strengths and limitations. This article explores the provides the opportunity important factors to consider when choosing which risk calculator to start treatments aimed at reducing CVD risk and to use, and outlines the pros and cons of calculators you may have means that therapies are access to in your practice. provided fairly. any people at increased risk of account if two different calculators give very 2. Risk calculators vary in their accuracy at cardiovascular disease (CVD) different estimates. It is also helpful to note predicting risk for an M are easy to identify because they that some calculators only estimate the risk of individual, but there have high blood pressure, high cholesterol, dying from a CVD event (Conroy et al, 2003). appears to be reasonable diabetes or smoke cigarettes. Estimating CVD agreement between them risk allows the identification of individuals Which calculator should I choose? in ranking individuals according to risk. without major risk factors who are actually Are the people in the calculator like at high risk. Identifying such people provides my patients? 3. Risk calculators help to the opportunity to start treatments aimed at Risk calculators are usually created from data identify individuals at high CVD risk allowing reducing CVD risk and means that therapies collected from clinical trials or epidemiological healthcare professionals are provided fairly. studies. How well the individuals from the to initiate risk reduction original study reflect your own patients will strategies. What constitutes CVD? give you an idea as to whether this is the Key words Each calculator has a slightly different way most appropriate calculator to use (Viljoen, of defining a CVD event, and hence CVD 2008). For example, the very commonly used - Cardiovascular disease - Macrovascular disease risk. The events that are usually included are Framingham equations (Anderson et al, 1991; - Risk assessment myocardial infarction, stroke, and death from D’Agostino et al, 2008), recommended by NICE - Type 2 diabetes a myocardial infarction or stroke. Additionally, (2006) and found inside the back cover of the some calculators include angina, transient British National Formulary (Joint Formulary ischaemic attack, intermittent claudication, Committee, 2009) were developed using data Hermione Price is heart failure, coronary artery bypass grafting collected from the Framingham studies. Clinical Research Fellow and percutaneous coronary angioplasty Framingham is a town in Massachusetts at the Diabetes Trials (Hippisley-Cox et al, 2007; D’Agostino et al, in the US and has a mainly affluent White Unit, Oxford Centre for Diabetes, Endocrinology 2008). Essentially, the broader the definition population. These equations may therefore be and Metabolism, University of CVD, the more likely it is that an individual less applicable if your patients live in an area of Oxford. will have an event. This should be taken into with high levels of deprivation or if you have 218 Diabetes & Primary Care Vol 11 No 4 2009 Cardiovascular disease risk estimation: Why, when and how? a high proportion of individuals from ethnic included in the study from which the calculator Page points minority groups. is developed. For example, in the Framingham 1. To be sure that a risk In addition, all studies have inclusion and study only 428 of the 8491 individuals included calculator is applicable exclusion criteria. It may not be appropriate to in the calculator had diabetes. to most people it should use a calculator to estimate risk for a 45-year-old have been shown to if the study from which the calculator is derived Including more risk factors does not mean provide reliable or “externally validated” only included people over the age of 50. improved accuracy estimates for many Crude risk estimates can be made using simple different individuals. Is the calculator reliable? measurements from the clinic, for example 2. If large quantities of data To be sure that a risk calculator is applicable waist circumference or BMI (Dalton et al, are imputed then this to most people it should have been shown 2003). Including additional measures, such may make you feel uneasy to provide reliable or “externally validated” as blood pressure and smoking status, and about using a particular estimates for many different individuals. For routine biochemical values, such as low-density calculator. example, its results should have been tested lipoprotein (LDL) and high-density lipoprotein 3. Although markers of and validated in individuals from different (HDL) cholesterol, can improve the accuracy of obesity are often included ethnic groups, of different ages and of different these estimates (Anderson et al, 1991). However, in cardiovascular disease socioeconomic status. many risk calculators attempt to increase the risk equations in addition to routine biochemistry Many risk calculators only test their accuracy of their estimates further by including and blood pressure, when estimates by “internal validation”, using a more and more novel risk factors (Woodward combined with these subset of the population from which the et al, 2007; Hippisley-Cox et al, 2008a). risk factors, obesity no calculator was developed to verify the results. Interestingly, including additional risk factors longer contributes to This does not ensure that the result will apply rarely improves the accuracy of risk estimates risk estimation. in a population that is different to the one any further. In addition, it can make many of 4. Framingham equations from which it was developed. these risk calculators redundant, as novel risk form the basis of many factors, including highly sensitive C-reactive commonly used CVD risk prediction charts, Was enough information available to produce protein levels or measures of carotid artery including the New a reliable calculator? calcification, are not readily available in routine Zealand CVD risk tables. All studies have inclusion and exclusion clinical practice (Ridker et al, 2002; Newman The equations are based criteria for participants, and few studies will et al, 2008). on a population sample be able to collect every piece of information on Although markers of obesity are often of 8491 adults aged 30–74 years. every individual taking part. It is important, included in CVD risk equations in addition therefore, to know how much information was to routine biochemistry and blood pressure missing and what the creators of the calculator (Balkau et al, 2004; Cederholm et al, did about this. 2008), when combined with these risk Missing information will often be imputed factors, obesity no longer contributes to risk (Hippisley-Cox et al, 2007). This is a statistical estimation. This is thought to be because the technique to replace missing data with combined effects of other CVD risk factors “invented” data based on what information outweighs the risk due to obesity (Coleman is available. If large quantities of data are and Holman, 2007). imputed then this may make you feel uneasy about using a particular calculator. Population-based risk calculators Framingham risk equations What about people with diabetes? Data from the landmark Framingham Heart Individuals with diabetes have a two- to four- Study and Framingham Offspring Study fold greater risk of CVD than those without have been used to update the well-established diabetes (Stamler et al, 1993). This increased Framingham risk equations (Anderson et al, risk is not reflected by all risk calculators 1991; D’Agostino et al, 2008). Framingham (Coleman et al, 2007a). This can occur if only equations form the basis of many commonly a small number of individuals with diabetes are used CVD risk prediction charts, including Diabetes & Primary Care Vol 11 No 4 2009 219 Cardiovascular disease risk estimation: Why, when and how? Page points the New Zealand CVD risk tables (Baker et al, SCORE also only includes a limited number 1. Framingham only 2000). The equations are based on a population of risk factors (age, sex, total cholesterol or total included a small sample of 8491 adults aged 30–74 years. HDL–cholesterol ratio, smoking status and number of individuals The advantage of Framingham over other systolic blood pressure). While these are clearly with diabetes (n=428), population study-derived risk equations is that the most important CVD risk factors, other key and the duration of data were extremely well collected and CVD factors such as diabetes and ethnicity are absent. known diabetes or other measures of disease events were meticulously assessed. The accuracy of the SCORE equations Disadvantages include the dominance of has been tested in a large Austrian cohort of severity such as HbA1c are not included. the Caucasian population and the relatively over 44 000 people drawn from the general 2. The Systematic Coronary high socioeconomic status of the participants. population (Ulmer et al, 2005). Of these, 487 Risk Evaluation (SCORE) The Framingham investigators realised their died from a CVD event. SCORE predicted risk equation was population was not ethnically diverse and that 666 CVD deaths should have occurred. developed to estimate the have now tested their equations in other However, SCORE did correctly identify those 10-year risk of dying from ethnic groups, including African Americans, most likely to die from a CVD cause. a cardiovascular disease (CVD) event in the Native Americans, Japanese American Unfortunately, the studies used did not general population only. men and Hispanic men (D’Agostino et al, have a standard method of recording diabetes 2001).