Bioelectrical Impedance Analysis

Bioelectrical impedance analysis estimates body composition by passing a small alternating current through the body and measuring the opposition to flow. Lean tissue, rich in water and electrolytes, conducts current well. Fat conducts poorly. Impedance therefore relates to total body water, from which fat free mass and fat mass are estimated by prediction equations.

The method is non-invasive, quick and portable. Its accuracy depends on hydration and on equations that were derived in specific populations.

Usage. It is used in nutritional assessment, sarcopenia evaluation, monitoring in kidney disease and dialysis, and research.

Indications. Assessment of fat mass and fat free mass, monitoring of nutritional status, evaluation of fluid distribution, and screening for low muscle mass.

Preparation and technique. The patient is measured after a fast of several hours and an empty bladder, with no vigorous exercise or alcohol in the preceding 12 to 24 hours. Electrodes are placed on the hand and foot, or the patient stands on a platform with hand grips. Single frequency devices estimate total body water. Multi-frequency devices also separate extracellular from intracellular water.

Contraindications and precautions. Pacemakers, implantable defibrillators and other active implants are usually a reason to avoid the test, unless the manufacturer permits it. Pregnancy is a relative contraindication. Metal implants alter the current path.

Interpretation.

• Fat mass and fat free mass are reported as kilograms and percentages, and compared with age- and sex specific reference values.
• Phase angle, derived from multi-frequency measurements, reflects cell membrane integrity and is lower in malnutrition, sarcopenia and chronic illness.
• Extracellular to total water ratio rises with fluid overload and inflammation.

Causes of misleading results. Dehydration or fluid overload, recent meals or exercise, oedema, ascites, extremes of body size, and use of equations from a different population.

Critical values and findings needing immediate action. A markedly low phase angle in a patient with weight loss or illness warrants prompt nutritional review.

Limitations and artefacts. Hydration changes alter results within hours. Ethnic differences in body composition limit the accuracy of general equations. Agreement with DEXA is moderate and varies between devices.

Follow up tests. DEXA body composition, handgrip strength, serum albumin and C-reactive protein, and anthropometry.

Special groups. Children need age specific equations. Older adults with low muscle mass need device specific cut offs for sarcopenia. Dialysis patients are measured at a fixed point in the cycle.