obesity is a serious health problem , which may range from a simple complaint of disability to premature death . in many epidemiological studies , the body mass index(bmi 30 kg / m ) or percentage of body fat ( pbf > 25% in men and > 35% in women ) the pbf is measured by several methods , such as magnetic resonance imaging ( mri ) , bioelectrical impedance analysis ( bia ) , and x - ray . the world health organization ( who ) , based on a 2004 study of 3,378 adults in southern iran , reported 21 to 25 kg / m as the range of the bmi and that 8% of women and 2.5% of men were obese without reporting the suitable cutoff points . mirzazadeh et al . conducted a meta - analysis of published reports in iran , including proceedings of professional meetings , books , journal articles , and estimated the prevalence of obesity in men and women to be 12.9% and 26.2% , respectively . in some iranian articles , suitable bmi cutoff points for detecting obesity for example , in one , it is reported to be 26.6 kg / m based on data from 206 elderly . in another study , data from 4,756 diabetic people were used to estimate the cutoff points as 24.8 kg / m and 26.3 kg / m for males and females , respectively . in the review of the literature , we identified iranian studies in which the bia had been used to measure body composition in samples ranging from 25 to 300 people . some reported that the bmi was accurate in detecting obesity while others concluded that it was not an accurate indicator of obesity . we also found non - iranian studies which supported the pros and cons of employing the bmi to detect obesity . gallagher classified individuals based on the pbf for males and females in various age groups , namely , 18 , 19 , 20 - 39 , 40 - 59 , and > 59 years old . a systematic review of the literature showed no published iranian studies using these age groups and no international studies focusing on either the accuracy of gallagher s classification or its comparison with the bmi . the purpose of this study was twofold , ( i ) to examine the comparability of the bmi and gallagher s classification in diagnosing obesity , and ( ii ) to determine the suitable bmi cutoff points for detecting obesity in a large sample of iranians , using the bia method . the significance of the study was that it compared two popular methods in detecting obesity , which had not already been done . the permission to conduct the study and the ethics approval were obtained from the office of vice - chancellor for research and technology at hormozgan university of medical sciences ( # 6-hec-88 - 2 - 16 , 6 may 2009 ) . the prevalence of obesity in the iranian adult population is reported as 21.50% , which we used to estimate the initial sample size at the 0.05 level of significance for testing non - directional hypotheses . however , due to the duration of the investigation ( 2009 - 2014 ) , 23,300 individuals who visited a health and diet therapy center in bandar abbas , iran , and agreed to participate in the study were recruited . those who were not at least 18 years old , had pacemakers , were pregnant , and were hospitalized at least 3 months prior to entering the study were excluded . in total , 20,163 adults met the inclusion criteria and their data were used in the study . this device uses the frequency range between 50 khz and 250 khz , and utilizes the method of direct segmental multi - frequency bia . with a high level of accuracy , this method enabled us to measure various body composition indices ( i.e. , the bmi , waist to hip ratio , percentage of body fat , total body fat , proteins , minerals , soft lean mass , fat free mass , muscle quantity , lean body mass , total body water , total energy expenditure , basal metabolic rate , fat - trunk , and muscle - trunk ) . the standard positions of outer and inner electrodes on the right hand and foot were employed for measuring the whole body impedance . the device was explained to all research participants and trained technicians were in charge of all measurements . the height ( cm ) was measured to the nearest 0.50 cm by a stadiometer . weight ( kg ) was divided by the squared height ( cm ) to measure the bmi , which was used to classify participants into four groups , ( i ) underweight ( under 18.50 ) , ( ii ) normal weight ( 18.50 to 24.99 ) , ( iii ) overweight ( 25.00 to 29.99 ) , and ( iv ) obese ( 30.00 and higher ) . the participants were also classified into four groups ( underweight , normal weight , overweight , and obese ) using gallagher s classification and subdivided the sample by sex and age . table 1 shows the classification of the participants based on the pbf using the gallagher s classification . distribution of participants based on gallagher s classification pbf : percentage of body fat the who gold standards were used to determine the accuracy of the bmi and gallagher s classification in detecting obesity . as noted earlier , pbf > 25% in men and > 35% in women indicate obesity . in the bmi and gallagher s classifications , all individuals in the subgroups of underweight , normal weight , and overweight were classified as non - obese and the rest were considered obese . for the purpose of the study , sensitivity , specificity , positive predictive value ( ppv ) and negative predictive value ( npv ) were computed using contingency tables . receiver - operating characteristic ( roc ) curves were used to describe the diagnostic performance of screening test in terms of diagnostic accuracy or the ability to classify participants correctly into clinically relevant subgroups , as defined by a reference test . version 22 of the statistical package for the social sciences ( spss ) was used for the purpose of data manipulation and analysis , which included contingency tables , linear regression , and hanely formula to compare the two roc curves . the participants were selected from 14,270 ( 70.80% ) women and 5,893 ( 29.20% ) men aged between 18 and 85 years . prevalence of obesity was estimated at 51% based on the gold standard 38.5% based on bmi and 27.4% by gallagher s classification . based on the gold standard 49.9% and 54.4% of women and men respectively , were classified as obese . results showed that the bmi was not suitable for detecting obesity among males . in this category , the following ranges were calculated : sensitivity ( 51.4% to 80% ) , specificity ( 89.4% to 100% ) , ppv ( 88.7% to 100% ) , and npv ( 53.2% to 93.9% ) . in the 20 - 39 , 40 - 59 , and 60 - 81 age groups , the bmi classified 34% , 45% , and 49% of male obese individuals as non - obese , respectively . the following ranges were obtained : sensitivity ( 41.1% to 100% ) , specificity ( 99.8% to 100% ) , ppv ( 99.8% to 100% ) , and npv ( 51.2% to 100% ) . in the 18 and 19 age groups , sensitivity , specificity , npv , and ppv were 100% . in the 20 - 40 age group in the 20 - 39 years age group , 9% , in 40 - 59 years age group , 46% , and in greater than 60 age groups , 59% of obese individuals were wrongly classified as non - obese . thus , it was concluded that the gallagher s classification is better than the bmi for detecting obesity in men with the exception of those who are older than 59 years ( table 2 ) . sensitivity , specificity , ppv , and npv based on the bmi and gallagher s criteria for overweight and obese men bmi : body mass index ; ppv : positive predict value ; npv : negative predictive value the number of women who were classified as obese by the bmi was 5,544 ( 38.9% ) . the bmi classified 25.7% of the obese women as non - obese , which led to a conclusion that it is suitable for detecting obesity in females . in this category , the following ranges were obtained : sensitivity ( 70.6% to 84.2% ) , specificity ( 92.8% to 98.3% ) , ppv ( 94.1% to 97.9% ) , and npv ( 56.1% to 93.5% ) . in the five age groups , the bmi classified 16% , 23% , 23% , 27% and 29% of obese womens as non - obese , respectively . based on this indicator the sensitivity ranged from 27.9% to 86.8% , and the npv s range was 35.2% to 94.7% . sensitivity and npv decreased as age increased , which was consistent with the bmi s findings . the range of decreases in sensitivity , based on the bmi and gallagher , were 13.6% and 58.9% , respectively . beyond 19 years of age , based on the gallagher s and the bmi , 55 - 72% and 29 - 33% of the obese individuals were wrongly classified as overweight , respectively . results showed that the bmi is better than gallagher in classifying the women ( table 3 ) . sensitivity , specificity , ppv , and npv based on the bmi and gallagher s criteria for overweight and obese women bmi : body mass index ; ppv : positive predict value ; npv : negative predictive value gallagher s classification can correctly classify men and women who are less than 40 and 20 years old , respectively . in both classifications , an increase in age was associated with a decrease in sensitivity and the npv . in all participants , a suitable cutoff point for bmi in detecting obesity was 27.70 kg / m ( sensitivity=88% , specificity=88.6% ) . among females , the suitable cutoff point was also found to be 27.7 kg / m ( sensitivity=92% , specificity=90% ) and among males , it was 27.3 kg / m ( sensitivity=84% , specificity=84% ) . the comparison of the two roc curves , using the hanely formula , showed no statistically significant differences between these cutoff points ( figure 1 ) . receiver operating characteristic for body mass index to detect obesity in all participants , women and men . in this category , the following ranges were calculated : sensitivity ( 51.4% to 80% ) , specificity ( 89.4% to 100% ) , ppv ( 88.7% to 100% ) , and npv ( 53.2% to 93.9% ) . in the 20 - 39 , 40 - 59 , and 60 - 81 age groups , the bmi classified 34% , 45% , and 49% of male obese individuals as non - obese , respectively . the following ranges were obtained : sensitivity ( 41.1% to 100% ) , specificity ( 99.8% to 100% ) , ppv ( 99.8% to 100% ) , and npv ( 51.2% to 100% ) . in the 18 and 19 age groups , sensitivity , specificity , npv , and ppv were 100% . in the 20 - 40 age group all ranges were between 90.8% and 100% . beyond 39 years of age , sensitivity and npv decreased as age increased . in the 20 - 39 years age group , 9% , in 40 - 59 years age group , 46% , and in greater than 60 age groups , 59% of obese individuals were wrongly classified as non - obese . thus , it was concluded that the gallagher s classification is better than the bmi for detecting obesity in men with the exception of those who are older than 59 years ( table 2 ) . sensitivity , specificity , ppv , and npv based on the bmi and gallagher s criteria for overweight and obese men bmi : body mass index ; ppv : positive predict value ; npv : negative predictive value the number of women who were classified as obese by the bmi was 5,544 ( 38.9% ) . the bmi classified 25.7% of the obese women as non - obese , which led to a conclusion that it is suitable for detecting obesity in females . in this category , the following ranges were obtained : sensitivity ( 70.6% to 84.2% ) , specificity ( 92.8% to 98.3% ) , ppv ( 94.1% to 97.9% ) , and npv ( 56.1% to 93.5% ) . sensitivity and npv decreased as age increased . in the five age groups , the bmi classified 16% , 23% , 23% , 27% and 29% of obese womens as non - obese , respectively . the sensitivity ranged from 27.9% to 86.8% , and the npv s range was 35.2% to 94.7% . sensitivity and npv decreased as age increased , which was consistent with the bmi s findings . the range of decreases in sensitivity , based on the bmi and gallagher , were 13.6% and 58.9% , respectively . beyond 19 years of age , based on the gallagher s and the bmi , 55 - 72% and 29 - 33% of the obese individuals were wrongly classified as overweight , respectively . results showed that the bmi is better than gallagher in classifying the women ( table 3 ) . sensitivity , specificity , ppv , and npv based on the bmi and gallagher s criteria for overweight and obese women bmi : body mass index ; ppv : positive predict value ; npv : negative predictive value gallagher s classification can correctly classify men and women who are less than 40 and 20 years old , respectively . in both classifications , an increase in age was associated with a decrease in sensitivity and the npv . in all participants , a suitable cutoff point for bmi in detecting obesity was 27.70 kg / m ( sensitivity=88% , specificity=88.6% ) . among females , the suitable cutoff point was also found to be 27.7 kg / m ( sensitivity=92% , specificity=90% ) and among males , it was 27.3 kg / m ( sensitivity=84% , specificity=84% ) . the comparison of the two roc curves , using the hanely formula , showed no statistically significant differences between these cutoff points ( figure 1 ) . receiver operating characteristic for body mass index to detect obesity in all participants , women and men . the aim of the study was to examine the comparability of the bmi and gallagher s classifications in detecting obesity in a large sample of iranian adults . a systematic review of the literature showed that the accuracy of gallagher s classification and its comparison with the bmi classification had not been adequately investigated . results showed that gallagher s classification is better than the bmi in detecting obesity in men younger than 40 years old ; beyond this age , both methods wrongly classified nearly half of the obese males as non - obese . the bmi was better than gallagher s classification in detecting obesity in females and an increase in age was associated with a decrease in the accuracy of both techniques , especially in gallagher s . the bmi correctly classified 70.8% of the obese persons , suggesting that it is a moderate indicator of obesity , which is supported by other studies . reported that the bmi accurately diagnosed excess adiposity in 84% of those participated in their study . in our study , the bmi and gallagher s criteria correctly identified 92.4% and 99% of males , and 96.5% and 100% of females as being non - obese , respectively . although the bmi is a good indicator for detecting non - obese individuals , but gallagher s classification is better than the bmi in classifying non - obese males and females . nearly 74.3% and 42% of females , and 63% and 79.4% of males were correctly classified as being obese by the gallagher s classification and the bmi , respectively . neovius reported low sensitivity and high specificity for the bmi in both sexes with the average age of 15 years . in spite of age differences , the results of this study are similar to those reported by neovius . based on the bmi , among females and males 16%-29% and 20%-49% of obese individuals these findings suggest that the bmi is a good indicator of obesity in females but not males . accuracy of the bmi in both females and males increased after the age of 19 years . coe et al . reported that the accuracy of the bmi decreased in females and no apparent trend was noted in males . it contradicts the results of this study , which could be due to age , ethnicity , and sample size differences . the correlations between the bmi and pbf were 0.86 and 0.91 in men and women , respectively ; similar to those reported by heyadari , et al . thus , it was concluded that the bmi is a good predictor of the pbf but not adequate in diagnosing excess adiposity , especially in males as supported by romero et al . and contradicted in other studies . the contradiction could be due to the selection of individuals , race , and sample size . the bmi among males had 9.1% false positive and 32.4% false negative , which were similar to a study by jitnarin et al . , who studied firefighters between the ages of 20 and 62 years . in short , there are errors in the identification of obese males with the bmi ; however , it is more suitable for females . in males under the age of 40 and females who are less than 20 years old , gallagher s classification is a better determinant of obesity than the bmi . reported a paradox in the findings of elderly patients with coronary heart disease with gallagher s classification by reporting the highest mortality in the underweight and normal groups and the lowest mortality in the overweight group . in our study , more than 46% of the male obese and 62% obese females older than 39 years were wrongly classified as overweight by this indicator ; thus , we concluded that it is not a good indicator to distinguish between older overweight and obese individulas , which made schutter s paradox questionable . a systematic comparison of all results showed that gallagher s classification is better in detecting obesity in men , especially among those who are less than 40 years old . on the other hand , the statement by rothman ,  the use of bmi as an indicator of obesity can introduce misclassification problems that may result in important bias in estimating the effects related to obesity  is confirmed . in spite of the large sample , it should be noted that the participants were recruited from southern iran and they do not represent the whole population of the country . we recommend replication of the study in other regions of iran , which could enhance the generalizability of the results . gallagher s classification is recommended for non - obese males and females and obese males . additionally , for the iranian population , it is recommended to employ 27.7 kg / m as the cutoff point for detecting obesity among women and men , 27.7 kg / m for women , and 27.3 kg / m for men . the results of the study would enlighten scientists to consider other methods in examining body conditions rather than the status quo .