although dualenergy xray absorptiometry ( dxa ) , computed tomography and magnetic resonance imaging ( mri ) are considered reference methods for identifying low skeletal muscle mass in elderly persons and patients with chronic diseases , access to these instruments may be limited in clinical practice .
for this reason , bioelectrical impedance analysis ( bia ) can be a useful tool to assess skeletal muscle mass , and three available consensus statements ( european working group on sarcopenia in older people , asian working group for sarcopenia and international consensus for cancer cachexia)1 , 2 , 3 accept bia as an option for identifying sarcopenia and cachexia .
however , bia , differently from the other body composition tools , does not actually measure a specific body component .
body composition assessment from bia relies on a calibration equation developed using a reference method such as dxa , computed tomography or mri .
early bia systems employed fatfree mass ( ffm ) prediction equations developed using traditional twocompartment reference methods such as underwater weighing or total body water . more recently
, other more sophisticated imaging methods have been used to develop bia prediction formulas , and new equations were developed to identify body components beyond that of ffm and related % fat . within the sarcopenia and cachexia areas ,
bia is now used to estimate several components that represent  muscularity. although these terms are often used as synonymous to ffm , they may not actually represent this component . depending on which body composition technique
was used to develop the bia system 's equation , compartments different from ffm are typically estimated .
this has led to some confusion in the literature , and we now review some of these measures with the aim of clarifying the role in patient assessment .
janssen et al . 
 4 validated bia against skeletal muscle mass ( sm ) obtained from mri , and
usually , 7375% of total body sm is located in the limbs and represents appendicular sm .
the same authors defined sm index ( smi ) as a % of total body mass ( sm / body weight , % ) x 100 , and smi is expressed in % units .
low smi was defined as a smi below one standard deviation of young adult values according to the data from the third national health and nutrition examination survey ( nhanes iii).5 the cutoff values suggested from this study were < 37% and < 31.5% ( class i and ii sarcopenia for men ) and < 27.6% and < 22.1% ( class i and ii sarcopenia for women ) .
another definition for smi was created some years later , considering smi as sm ( in kg , obtained from the same bia equation ) adjusted for the squared height ( sm / height , kg / m ) .
based in physical disability risk also assessed in the nhanes iii elderly population , the authors defined the usual cutoff used to define sarcopenia with smi : moderate sarcopenia when smi is between 8.51 and 10.75  kg / m ( men ) or 5.76 and 6.75  kg / m ( women ) and severe sarcopenia when smi is 8.50  kg / m ( men ) or 5.75  kg / m ( women).6 these cutoff values are used in the european working group on sarcopenia in older people consensus , when absolute sm is estimated from bia.2 
  in the cachexia consensus statement , fearon et al
. 
 3 proposed three criteria for diagnosing cancer cachexia : 
 weight loss > 5% over past 6  months ( in the absence of simple starvation ) ; orbody mass index  <  20  kg / m and any degree of weight loss > 2% ; orappendicular skeletal muscle index consistent with sarcopenia . 
 
weight loss > 5% over past 6  months ( in the absence of simple starvation ) ; or body mass index  < 
20  kg / m and any degree of weight loss > 2% ; or appendicular skeletal muscle index consistent with sarcopenia .
sarcopenia can be assessed by several techniques , as described in the paper , including bia .
the authors used the term whole body ffm index without bone and defined the cutoff as < 14.6  kg / m ( men ) and < 11.4  kg / m ( women).3 the term whole body ffm index without bone is not exactly the same thing as sm , estimated from bia as described in the cited reference.5 there was likely confusion here with the term lean tissue without bone ( compartment obtained from dxa ) . based in the study from janssen et al . , the cutoff values cited by fearon et al . 

3 are much higher than those used to define low muscularity in the sarcopenia consensus .
this would lead to a misclassification of subjects according to smi using bia , diagnosing almost everyone as sarcopenic , and consequently , cachectic .
appendicular skeletal muscle mass ( asmm ) is another term given to the assessed  soft lean appendicular tissue ( the fat and bonemineral free tissue , obtained from dxa ) assessed in the four limbs .
it should be a little larger than the actual appendicular skeletal muscle mass , because the skin and connective tissue are included in this measurement .
this compartment can be estimated when a bia equation is derived from a validation study where asm was assessed from dxa.7 , 8 , 9 , 10 asmm can also be normalized in relation to height , giving the appendicular smi ( asmi  =  assm / height 
 , kg / m ) .
different equations were developed from different populations ( adults , elderly , caucasian and asian subjects ) .
only the asian consensus suggests cutoff values of asmi by using bia ( 7.0  kg / m in men and 5.7  kg / m).1 these values were established from 1719 young healthy japanese volunteers , and asmi was defined as the sum of the muscle mass of the arms and legs divided by height in meters squared , assessed directly by using segmental bia.11 , 12 

recently , this new term was introduced to describe exactly the same compartment as appendicular sm from dxa : the lean mass without bone and fat assessed by dxa in the four limbs.13 the authors obtained two different equations to estimate appendicular lean mass using bia , depending of the type of dxa systems used in the validation ( ge lunar or hologic ) .
although these equations were developed from a sarcopenic population ( defined by the previously described janssen 's equation for bia ) obtained from a large european multicentre study ( provide nutritional intervention study ) , no cutoff values were shown , and the authors suggested that the reference values from hologic and lunar should be used to characterize sarcopenia .
bioelectrical impedance analysis can estimate different compartments that are used to define sarcopenia and cachexia , as described earlier . as they were estimated using different bia equations , they are valid for the same population from the validation study , using the same bia device , and the subjects should be classified according to the adequate suggested cutoff values .
for this reason , it is important to standardize not only the terminology employed to define low muscularity and sarcopenia but also the cutoff values for diagnostic purposes in each population .
the terms and cutoff values suggested in the cancer cachexia international consensus to diagnose sarcopenia by using bia should be reviewed , as an overestimation of cachexia can result with the actual values .
further research may show the need for specific cutoff values to define sarcopenia in different populations .
maria cristina gonzalez and steven b. heymsfield declare that they have no conflict of interest .
 abstractas reference methods are not available for identifying low skeletal muscle mass in clinical practice , the european group on sarcopenia in older people the asian working group for sarcopenia and the international consensus for cancer cachexia guidelines accept bioelectrical impedance analysis ( bia ) as an option for sarcopenia and cachexia assessment . using different bia equations , several components that represent  muscularity can be assessed . 
 total skeletal muscle mass or appendicular skeletal muscle mass normalized in relation to height ( skeletal muscle mass index or appendicular skeletal muscle index , respectively ) is the most common term used in the consensus . 
 these terms are similar , but they should not be used as synonymous . both terms can be used to define sarcopenia , but adequate equations and cutoff values should be used according to the studied population . however , there is a disagreement between the sarcopenia definition assessed by using bia from the european group on sarcopenia in older people and cachexia consensus , and this can lead to an overestimation of sarcopenia and , consequently , cachexia . 
 an effort should be made to standardize the terminology employed by the societies to define low muscularity and sarcopenia by using bia . 
 future validation studies may show the need for specific cutoff values for each population using this method . 
Bioelectrical impedance analysis for diagnosing sarcopenia and cachexia: what are we really estimating?
Total body skeletal muscle mass
Appendicular skeletal muscle mass
Appendicular lean mass
Can bioelectrical impedance analysis be used to identify sarcopenia/cachexia?
Conflict of interest