136 Chapter 7 Table 2 . In vivo techniques for measuring body composition in preterm born infants 12,13 Technique Method Application Body proportionality calculations Ponderal index (kg/m3), Body mass index (BMI) (kg/m2), Benn index (weight/lengthp), waist, mid-arm, and mid-thigh circumference, mid-arm/ head circumference ratio These indices are generally included in regression models to formulate predictive equations that estimate fat (free) mass (g) or fat (free) mass percentage Skinfold thickness measurements Skinfold thickness is measured with a caliper to estimate subcutaneous fat The absolute value of the skinfold at a particular site or the sum of skinfolds measured at different sites are generally included in regression models to formulate predictive equations that estimate fat (free) mass (g) or fat (free) mass percentage Isotope dilution Isotope distribution is measured in blood or urine. Total body water is calculated from the isotope distribution. Fat free mass (g) is calculated from total body water. Bioelectrical impedance analysis / Bioelectrical impedance spectroscopy The flow of electrical currents through tissue is measured Total body water is calculated from the resistance to the flow of electrical currents running to body tissue (Total body water = pL2/Z; p = volume resistivity in Ω-cm, L = length in cm, Z = impedance in Ω) Ultrasound Measures muscle area Estimates subcutaneous fat by subtracting the muscle cross sectional area from the total cross sectional area Magnetic Resonance Imaging Measures adipose tissue volume based on tissue specific energy absorption Measures fat mass (g) Dual-energy x-ray absorptiometry Measures fat mass and fat free mass based on tissue specific x-ray attenuation Measures fat (free) mass (g) Air displacement plethysmography Measures body volume based on air displacement in a test chamber Calculates fat mass (g) and fat mass percentage through age- and sexspecific density coefficients Data extraction was performed by these two reviewers. The data collected included the method and reference method, details on the study setting, methods and results. In case of any discrepancies between the two reviewers, the two reviewers came to agreement through discussion. Reviewers were not blinded for authors or journal details. Where needed, authors were contacted for clarification or additional information.
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