相位角与阻抗之比：生物阻抗光谱法产生的国家健康及营养调查IV和V参考割点

　　生物阻抗参数，如相位角（PA）和阻抗比（IR）可用作营养状况和临床结局的预测，但需要提供人群合适并匹配的参考数据，如年龄、性别、种族和体重指数（BMI），而目前美国人群的参考数据证据很有限。美国明尼苏达大学双城分校、巴西佩洛斯天主教大学等对美国健康及营养调查（NHANES）1999～2000，2001～2002和2003～2004年的数据进行分析，并探究PA、IR与无脂肪体重（FFMI）之间的关系，发现在确定PA和IR的参考数据时，种族是重要的影响因素，并且PA、IR与FFMI呈正相关，该研究为生物阻抗参数应用于美国人群的营养评估提供了证据。

JPEN J Parenter Enteral Nutr. 2016;40(4):125-127.

Phase Angle and Impedance Ratio: Bioimpedance Spectroscopy-Generated Reference Cut Points From National Health and Nutrition Examination Survey IV and V.

Adam Kuchnia; Levi Teigen; Abigail Cole; Urvashi Mulasi; Maria Cristina Gonzalez; Carrie Earthman.

University of Minnesota-Twin Cities, Minneapolis-St Paul, MN, USA; Catholic University of Pelotas, Pelotas, Brazil.

Purpose: Raw bioimpedance parameters—for example, 50-kHz phase angle (PA) and 200-kHz / 5-kHz impedance ratio (IR)—have been investigated as predictors of nutrition status and/or clinical outcomes. However, their validity as biomarkers depends on the availability of appropriate reference data. Previously, reference data have been categorized by age, sex, and/or body mass index (BMI) for German and Swiss (primarily Caucasian) populations. More limited data have been published for the U.S. population. Interstudy discrepancies, however, suggest that additional variables need to be accounted for to improve the accuracy of population reference values. Using a large and ethnically diverse data set, we aimed to identify additional variables and provide expanded bioimpedance reference data for the U.S. population.

Methods: The National Health and Nutrition Examination Survey (NHANES) is an ongoing compilation of studies conducted by the U.S. Centers for Disease Control and Prevention designed to monitor the nutrition status of the U.S. population. The NHANES IV and V data sets from 1999-2000, 2001-2002, and 2003-2004 were used for this analysis. A bioimpedance spectroscopy device (Hydra Model 4200; Xitron Technologies, San Diego, CA) was used for bioimpedance data collection only in persons aged 18-49 years. A subsample had dual-energy X-ray absorptiometry (DXA) data collected; in these individuals, PA and IR were further evaluated for their relationship to DXA-measured fat-free mass index (FFMI). Within ethnicity and sex, participants were further categorized by those falling ≤5th and within the 25th-50th percentiles for FFMI; mean PA and IR values were calculated for the FFMI groups. Available bioimpedance spectroscopy data were categorized by ethnicity; final groupings were Hispanic, white, black, and other.

Results: Multivariate analysis showed that PA and IR differed by BMI, age, sex, and ethnicity (n = 6237; R2 = 0.412, P < .0001). Suggested reference cut points for PA, stratified by age decade, ethnicity, and sex, in a similar format to previously published population data. Mean BMI ± SD is also provided for each category, given that sample size limitations prevented further delineation of the data by BMI. Mean FFMI correlated with PA and IR measures (r = 0.55 and r = -0.58, respectively; P < .0001). Within all categories, PA was lower and IR higher in the ≤5th percentile FFMI group vs the 25th-50th percentile group.

Conclusions: Ethnicity is an important variable that should be accounted for when determining population reference values for PA and IR. We have provided sex-, ethnicity-, and age decade-specific reference values for PA for use in future studies in U.S. populations. Interdevice differences are likely to be important contributors to variability across published population-specific reference data and, where possible, should be evaluated in future research. Ultimately, further validation with DXA-derived measures are necessary to determine if PA and IR are appropriate bedside diagnostic tools for the assessment of nutrition status in a clinical population.