Sensitivity and specifi city of the body mass index for the diagnosis of overweight/ obesity in elderly Sensibilidade e especifi cidade do índice de massa corporal no diagnóstico de sobrepeso/ obesidade em idosos

The aim of this article was to verify the sensitivity and specificity of the body mass index (BMI) cut-off points proposed by the World Health Organization (WHO) and the Nutrition Screening Initiative (NSI) for the diagnosis of obesity in the elderly. A cross-sectional study was made with 180 healthy elderly subjects from Florianópolis, Santa Catarina State, Brazil. Body fat percentage (%BF) was determined using DEXA (dual energy X-ray absorptiometry). The BMI cut-off point of the NSI offers better sensitivity and specificity for men (73.7% and 72.5% respectively). For women, the lower the cut-off point the better the sensitivity, with a BMI of 25kg/m2 (sensitivity of 76.3% and specificity of 100%) being the most accurate for diagnosing obesity in elderly women. The WHO cut-off point offered very low sensitivity (28.9%). The results of this investigation lead to the conclusion that the cut-off points proposed by the WHO and the ones adopted by the NSI and by Lipschitz are not good indicators of obesity for the elderly of either sex, since they offer low sensitivity. Body Mass Index; Sensitivity and Specificity; Overweight; Obesity; Aged Introduction The aging process causes nutritional, morphological and physiological modifications, such as reductions in height and body weight, reduction of fat-free mass and increase in fat mass 1,2,3. The increase in fat mass, particularly in the trunk region, occurs especially after the age of 45. It is related to the redistribution of fat mass, with a reduction of fat in the limbs being followed by an increase in trunk fat, due to hormonal factors 4,5. The aging process also affects men differently to women. Men have a smaller absolute loss of muscle mass when compared with elderly women. Elderly women, on the other hand, exhibit an increase in fat mass and a reduction in bone mass 6,7. Excessive fat mass may be defined as obesity, a chronic disease directly or indirectly related to other pathological situations which contribute to morbidity and mortality, such as cardiovascular, bone-muscular and neoplastic diseases 8. Nevertheless, the methods available for diagnosing obesity in the elderly are still not specific, considering the modifications in body composition which occur during the aging process. The most widely used method is the one proposed by the World Health Organization (WHO) 8 based on body mass index (BMI) for the adult population. According to this method, an individual is considered pre-obese when having a BMI equal to or above 25kg/m2 and below 30kg/m2 and obese ARTIGO ARTICLE Vasconcelos FAG et al. 1520 Cad. Saúde Pública, Rio de Janeiro, 26(8):1519-1527, ago, 2010 when having a BMI equal to or above 30kg/m2, regardless of age or sex. It is worth emphasizing that these cut-off points for BMI were originally established based on the risk of developing associated morbidities or chronic diseases such as hypertension, high blood cholesterol, type 2 diabetes, coronary heart disease, and other diseases 8. In the 1990s, another classification based on BMI was proposed for the diagnosis of obesity, adapted to the elderly population. This classification is used by the Nutrition Screening Initiative (NSI) 9 and adopted by Lipschitz 10, in which seniors with a BMI above 27kg/m2 are classified as overweight, while those with a BMI below 22kg/m2 are classified as thin. Lipschitz 10 says that the use of these values from the lesser mortality of the elderly in this BMI range, however, does not refer to changes of aging. Some limitations, however, have been identified when using these diagnostic procedures with the elderly population, including studies conducted in Brazil 11,12,13. Lohman 14 points out that BMI can be influenced by muscle mass, organs, bone structure and fat mass. Therefore, an individual with great quantities of fat free mass could exhibit the BMI of an obese person, but not have excessive fat mass. Similarly, an individual with small bone structure or reduced muscular mass, but with a great quantity of fat mass, could be misclassified as normal (eutrofic) when actually overweight 14. Therefore, when the distinct changes to the body that occur in aging are taken into consideration, it is possible that two people with the same BMI may actually have different percentage fat mass 15. This is why BMI should be specific for age groups and differentiated among populations and ethnic groups 16. On the other hand, Lohman 14 states that the cut-off points for the diagnosis of obesity based on percentage body fat for adult population, which places an individual at risk is above 25% of body fat for men and 32% for women. Considering the changes in body composition during aging and the importance of an accurate diagnosis of obesity in the elderly, the objective of this study was to verify the sensitivity and specificity of the BMI cut-off points proposed by WHO 8 and NSI 9 for diagnosing obesity in the elderly. Materials and methods This was a cross-sectional study. The sample was composed of 180 elderly subjects (60 men and 120 women), considered healthy or without evidence of serious chronic diseases, all of them living in the municipality of Florianópolis in the State of Santa Catarina, Southern Brazil, in 2005. Details about the development and design of the study have been described elsewhere 17,18. Briefly, the subjects were selected by invitation among participants of four elderly leisure groups. These groups were selected among people living close to the places where the measurements were carried out, thus avoiding major discomfort for the subjects. A total of 266 elderly subjects (115 men and 151 women) were contacted by telephone and interviewed. 86 subjects (57 men and 29 women) did not fulfill the criteria for inclusion in the sample since they presented symptoms of high blood pressure, had some type of cancer, amputation and pacemaker, were younger than 60 years, were taking diuretics, had a fixed prosthesis or were not interested in participating in the study. During the telephone contact, the subjects also responded to a questionnaire regarding demographic and socioeconomic data and the dates for the anthropometric, dietetic, bioelectrical impedance and dual energy X-ray absorptiometry (DEXA) measurements were also scheduled 17,18. Research was conducted according to the ethical principles regarding an individual’s autonomy, as per Resolution no. 196/96 of the National Health Council. The project was approved by the Human Research Ethics Committee at the Federal University of Santa Catarina (UFSC), under protocol no. 063/05. Anthropometric measures (weight and height) were taken during the morning (between 8 and 10am), at the Physical Effort Laboratory, part of the UFSC Sports Center. All data were collected by one person, (with error of measurement of between 0.24% and 2.62% 19), who had been given appropriate training in the techniques and standards employed 20. The BMI [weight (kg)/ height (m2)] was classified by using two criteria: (1) the WHO cut-off points 8, considering preobese those subjects with BMI above 25kg/m2 and below 30kg/m2; and obese those with BMI above 30 kg/m2; (2) the cut-off point adapted for the elderly and used by the NSI 9, whereby overweight individuals were considered to be those with a BMI above 27kg/m2. Percentage body fat and fat free mass were measured by whole-body scanning with a Lunar Prodigy DF + 14.319 radiation densitometer (GE Medical, Madison, USA) using the DPX-L software version 7.52.002 (SONITEC, Florianópolis, Brazil) for quantification. During evaluation, each senior wore just an apron, was barefoot without earrings, rings, dental prosthetics or other materials, and was immobilized in dorsal decubitus, keeping the arms and legs away from BODY MASS INDEX FOR DIAGNOSIS OF OVERWEIGHT/OBESITY IN ELDERLY 1521 Cad. Saúde Pública, Rio de Janeiro, 26(8):1519-1527, ago, 2010 the body. Each examination took around 10-15 minutes. The equipment was calibrated daily, according to the manufacturer’s instructions. Evaluations were conducted by a technician in medical radiology, at the Diagnostic Imaging Center (SONITEC), in the city of Florianópolis, during the afternoon, between 4pm and 6.30pm. Both anthropometric and DEXA data were collected in two different hours on the same day, due to the distance between the places for data collection and the schedules offered by the clinic. This does not generate a bias, since there is no comparison between different methods of body fat percentage and there is no significant change in body composition in a few hours. Data were analyzed using the SPSS, version 11.5 (SPSS Inc., Chicago, USA), adopting a level of significance of p < 0.05. Student’s t test for independent variables was used to compare age, weight, height, BMI and percentage of body fat between men and women and between obese and non-obese groups. The sensitivity and specificity of the different cut-off points for obesity diagnosis using BMI were estimated. These tests are described as methods that are theoretically capable of indicating the presence or absence of a given disease, with a certain chance of error. The probability of a diagnostic test producing a positive result, when the individual does indeed have a given disease, is called test sensitivity; and the probability of the test producing a negative result, when the individual does not have the disease, is called specificity 21. In order to estimate sensitivity and specificity, it was necessary to create tables of crossed tabulation comparing the different cut-off points to reference values, based on percentage of body fat. For this procedure the percentage of body fat obtained by DEXA was adopted as gold standard for the diagnosis of obesity, according to various investigations made with the elderly population 1,6,15,22,23. The values proposed by Lohman 14 for adult population, of 25% of body fat for men and 32% of body fat for women, was adopted as reference cut-offs. To determine the most adequate BMI cut-off points for the diagnosis of obesity in the elderly, a statistic procedure known as ROC (receiver operating characteristic) curve was employed. In this procedure, the area under the curve (AUC) can be used to quantify how well a predictor discriminates between individuals with and without a disease 24, with the disease in this case being obesity. The AUC was then calculated to determine the most sensitive and specific cut-off points for obesity in the population studied. Results Table 1 lists the physical characteristics of the elderly subjects in the form of descriptive statistics. The sample consisted of 180 healthy elderly subjects (60 men and 120 women) ranging in age from 60 to 81 years. Men had greater mean age, weight and height. The men and women had similar BMI values, but the difference between them was significant (p < 0.05). Women had higher percentages of body fat and had lower fat free mass than men, suggesting that sex can be an important variable in determining obesity cutoff points.