Evaluation of between-methods agreement of extracellular water measurements in adults and children.
BACKGROUND
Extracellular water (ECW), a relevant molecular level component for clinical assessment, is commonly obtained by 2 methods that rely on assumptions that may not be possible to test at the time the measurements are made.


OBJECTIVE
The aim of the current study was to evaluate the degree of agreement between ECW assessment by the sodium bromide dilution (ECW(NaBr)) and total body potassium (TBK; whole-body (40)K counting) to total body water (TBW; isotope dilution) methods (ECW(TBK-TBW)) in an ethnically mixed group of children and adults.


METHODS
ECW was measured with the ECW(NaBr) and ECW(TBK-TBW) methods in 526 white and African American males and females (86 nonobese children, 193 nonobese adults, and 247 obese adults). Fat mass was assessed with dual-energy X-ray absorptiometry. Multiple regression analysis was used to examine the variables related to between-ECW method differences.


RESULTS
Significant but generally small group mean (+/-SD) differences in ECW were found in the obese adults (1.28 +/- 2.54 kg) and children (-0.71 +/- 1.78 kg). The magnitude of the differences was related to mean ECW in obese adults, children, and nonobese adults, and the relations between these variables were modified by sex for nonobese adults. ECW differences were also dependent on age, weight, sex, and race or on interactions between these variables.


CONCLUSIONS
Overall, although good between-method agreement was found across the 3 groups, the degree of agreement varied according to subject characteristics, particularly at the extremes of ECW and body weight. We advance a possible mechanism that may link subject characteristics with the degree of agreement between ECW measurement methods and their underlying assumptions.
Silva
Analiza M
AM
New York Obesity Research Center, St Luke's-Roosevelt Hospital, Columbia University Institute of Human Nutrition, College of Physicians and Surgeons, New York, NY, USA.
Heymsfield
Steven B
SB
Gallagher
Dympna
D
Albu
Jeanine
J
Pi-Sunyer
Xavier F
XF
Pierson
Richard N
RN
Wang
Jack
J
Heshka
Stanley
S
Sardinha
Luis B
LB
Wang
Zimian
Z
D015502:Absorptiometry, Photon; D000273:Adipose Tissue / Q000378:metabolism; D000328:Adult; D001741:Black or African American; D000367:Age Factors; D001823:Body Composition / Q000502:physiology*; D001834:Body Water / Q000378:metabolism*; D001965:Bromides / Q000032:analysis*; D002648:Child; D005006:Ethnicity; D005110:Extracellular Space; D005260:Female; D006801:Humans; D007201:Indicator Dilution Techniques; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000378:metabolism*; D011188:Potassium / Q000032:analysis*; D011197:Potassium Radioisotopes; D012044:Regression Analysis; D012737:Sex Factors; D017670:Sodium Compounds / Q000032:analysis*; D044465:White People
D001965:Bromides; D011197:Potassium Radioisotopes; D017670:Sodium Compounds; C027938:sodium bromide; D011188:Potassium