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Precision and Accuracy of Defining Obesity

The fifth item on the  disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  deals with issue of precision, accuracy, and reproducibility. Obviously, any definition of obesity that requires clinical assessment and clinical judgement will not have the precision, accuracy, or reproducibility of simply measuring height and weight. Thus, if we define obesity as the presence of abnormal or excess body fat that impairs health, we will necessarily have to deal with the issue of assessing health, which is not something that you can simply measure by stepping on a scale. Rather, because abnormal or excess fat can affect virtually every organ system as well as psycho-social well-being, we are going to be faced with a rather complex system of diagnosing who has obesity and who hasn’t. In fact, as the authors of the checklist point out, “…an appropriate gold standard will rarely be available and therefore, traditional measures of diagnostic test accuracy, such as sensitivity and specificity, will generally not be appropriate.” Both repeatability (agreement in identical conditions) as well as reproducibility (agreement across comparable conditions) may result from biological variability, analytical variability, and clinical judgement. The only way to test the reproducibility and precision will be to evaluate the use of the new definition in clinical practice and ultimately determining whether or not clinicians can reasonably agree on who has the condition and who doesn’t. While this may seem daunting to non-clinicians, let us remember that in clinical practice many diagnoses are dependent on clinical evaluations and clinical judgement, whereby experienced clinicians or specialists may perform better than the novice or the non-specialist (a good example is psychiatry, but there are countless other examples). Moreover, there will always be grey areas in “borderline” cases, where examiners may disagree on the exact result and only time will tell, who is right. Welcome to the messy world of clinical practice. Just because BMI is simpler, more precisely measured, and more reproducible, does not make it a better measure of diagnosing whether or not someone actually has a disease. After all it only makes sense that it will take a complex definition to diagnose a complex disease. @DrSharma Edmonton, AB

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Obesity Prognosis

Continuing in my discussion of the disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  I turn to the fourth item, which deals with issue of prognostic ability of changing the obesity definition? Indeed, as pointed out by the authors of the checklist, “The most important feature of a disease definition is its ability to accurately predict clinically meaningful outcomes.” This, of course is, where BMI fails hands down. As we have demonstrated using several large data sets, the ability of BMI to predict mortality is rather limited with almost no relationship between outcomes and BMIs, at least over a rather wide range of BMI levels. Contrast this to the power of predicting outcomes when using a definition that actually looks at the presence of health impairments, such as the Edmonton Obesity Staging System.  When applying this system, which takes into account the impact of excess weight on mental, physical, and functional health, the prognostic power ranges from virtually no risk (Stage 0) to halving your chances of being alive in 20 years (Stage 3). Thus, a definition of obesity that actually considers impairments in health have a far greater prognostic power than simply knowing someone’s BMI. If nothing else, this alone should be a reason to abandon BMI for a more meaningful definition of obesity, that actually considers health and not just size. @DrSharma Edmonton, AB

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Why Redefine Obesity?

The third item on the disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  pertains to the issue of why modify the disease definition at all? With obesity being increasingly recognize as a chronic disease, it should be evident to anyone, that the current BMI-based definition of obesity, although simple (or rather simplistic), would label a substantial number of individuals as “diseased”, who may be in rather good health and, therefore, very unlikely to benefit from any obesity treatments (overdiagnosis). On the other hand, the current BMI-based definition excludes a vast number of people, who may very well have health impairments attributable to abnormal or excess body fat, and may thus benefit from obesity treatments (underdiagnosis). Although there have been many suggestions for replacing BMI with other anthropometric measures (e.g. waist-to-hip ratio, ponderal index, abdominal sagittal diameter, etc.), none of these measures would guarantee that the individuals identified by such measures, would indeed have health impairments attributable to abnormal or excess weight – their sensitivity and specificity, although perhaps marginally better than BMI in identifying individuals with excess body fat, would still not pass the sniff-test for a reliable diagnostic test of an actual disease. In fact, given the diversity and heterogeneous nature of adipose tissue, even more precise measures of actual body composition (including sophisticated imaging techniques) would still not be enough to determine whether or not body fat in a given is in fact impairing health and warrants obesity treatment. In contrast, a definition of obesity that requires the actual demonstration of health impairments (likely) attributable to abnormal or excess body fat, via a clinical assessment, would ensure that obesity is only diagnosed in individuals, who actually have a health problem and would therefore likely benefit from obesity treatments. This may well include individuals below the current BMI cut-off. Thus, continuing to use BMI (or any other anthropometric measure or more sophisticated estimate of body fat) is simply not an option if we are serious about calling obesity a disease. @DrSharma Ottawa, ON

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Impact Of Changing Obesity Definition on Prevalence

The second item on the disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  pertains to the issue of how a proposed new definition would alter the prevalence of the disease. As indicated in the name of the working group that came up with this checklist, their primary concern is over-diagnosis or “diagnosis-creep”, as often disease modifications tend to increase the number of people covered under said new diagnosis. So what is the implication for prevalence of obesity if we move from a definition based on BMI to one based on an actual impairment of health? Fortunately, we have some data on this, including our own studies on the Edmonton Obesity Staging System, which ranks individuals based on the presence of obesity related impairments in mental, physical, and/or functional health. Based on varying estimates, anywhere between 5-15% of individuals with a BMI over 30 would be considered to be rather healthy with no or minimal health risks. These people would need to be excluded, if obesity was defined as the presence of abnormal or excess body fat that impairs health (they may at best be considered to have “pre-obesity”). This would slightly reduce the number of people considered to have obesity (especially in the BMI 30-35 range). On the other hand, an estimated 40-50% of individuals in the BMI 25-30 range, would actually have significant health problems at least in part attributable to their excess weight, and these individuals may potentially benefit from obesity treatments. Thus, such a change in definition would very substantially increase the number of individuals considered to have obesity. This, of course is something that needs to be carefully considered, as it would clearly have implications for obesity treatment in a significant number of individuals, who at this time would not meet the criteria for obesity management. Let us, however, remember that one would still need to demonstrate significant benefit of treatment in these newly classified individuals. before expanding the indication of existing obesity treatments to these individuals. @DrSharma Edmonton, AB

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Obesity Definition Then And Now

The first item on the disease definition modification checklist developed by the Guidelines International Network (G-I-N) Preventing Overdiagnosis Working Group published in JAMA Internal Medicine,  pertains to the issue of how a proposed new definition would differ from the existing definition. As authors are well aware, the current definition that is widely used to define obesity is based on BMI, a simple anthropometric measure calculated from body height and weight – a great measure of size, not such a great measure of health. In contrast, the proposed definition of obesity, where obesity is defined as the presence of abnormal or excess fat that impairs health, would require the actual assessment and demonstration of the presence of health impairments attributable to a given subject’s body fat. Thus, while anyone can currently “diagnose” obesity simply by entering height and weight into a BMI calculator and looking up the value on a BMI chart, the new definition would in fact require a full clinical assessment of an individual’s health. Such an assessment would need to look at both mental and physical health as well as overall well-being for issues that may be directly caused (or aggravated by) the presence of abnormal of excess body fat. This does in fact bring up the issue of how exactly you would define “abnormal” or “excess” body fat and, even more importantly, how you would establish a relationship between body fat and any health impairments in a given individual. While these issues would clearly need to be worked out, the face value of this approach should be evident in that it focusses on the issue of actual health impairments rather than an arbitrary BMI cut-off, above which everyone would be considered as having obesity. This of course raises a number of issues around definition precision and accuracy, which is another item on the checklist and will be discussed in a future post. @DrSharma Edmonton, AB    

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