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phase angle

Phase angle is an indicator of cellular health and integrity.
Research in humans has shown that the relationship between phase angle and cellular health is increasing and nearly linear (1,2,3). A low phase angle is consistent with an inability of cells to store energy and an indication of breakdown in the selective permeability of cellular membranes. A high phase angle is consistent with large quantities of intact cell membranes and body cell mass.

Phase angle reflects the ratio of body cell mass to fat-free mass.
Phase angle is proportional to the ratio of reactance and resistance. Therefore, phase angle is proportional to the ratio of body cell mass to fat-free mass.

What causes the phase angle to increase?
 • An increase in body cell mass relative to fat-free mass.
 • An increase in fat-free mass relative to body weight.
 • Improving hydration of fat-free mass.

Phase angle is useful when comparing individuals.
Reactance along with the patient's weight indicates an absolute amount of body cell mass (BCM). Therefore, reactance is best applied when comparing test results in a single patient at different times. It is possible for two patients with exactly the same reactance (X) to have differing amounts of BCM in kilograms, depending upon the patient's weight.

However, since the phase angle indicates a proportion of BCM to FFM, any patient with a higher phase angle will always have a higher proportion of BCM than any other patient with a lower phase angle.

Phase angle does not include the effect of statistical regression.
Like body cell mass (BCM), the phase angle indicates the number of intact cell membranes. However, phase angle does not include the effect of statistical regression analysis. As a result, phase angle is a direct measurement of relative amounts of intact cellular membranes.

What exactly is the phase angle, anyway?
A bioimpedance analyzer applies a small 50 kilohertz alternating current to the body. If an oscilloscope were connected to the body, the phase angle appears as a small delay between the voltage waveform and the current waveform.

The period of each wave at 50 kilohertz is 20 microseconds. If, for example, the time delay is ten percent of the period, then the time delay is 2 microseconds. When expressed in units of time, it is said that the phase delay is 2 microseconds.

Another way of expressing this time delay is as a percentage of the entire wave period in degrees. Each complete wave period consists of 360 degrees. If the time delay is one-tenth the total period of the wave, it is equivalent to 36 degrees. When the time delay is expressed this way (in degrees of the total wave period), it is called the phase angle.

When electrical potential and current are illustrated sweeping around a circle instead of moving over time, the relationship between reactance, resistance, and phase angle is easier to see. This is shown below.

The range of phase angle in the human body is 1 to 20 degrees. The phase angle is the arctangent of (X/R).

1Kyle UG, et al. Fat-Free and Fat Mass Percentiles in 5225 Healthy Subjects Aged 15 to 98 Years. Nutrition, 17:534-541, 2001.
2Mattar J, et al. Application of total body bioimpedance to the critically ill patient. New Horizons 1995, Volume 4, No, 4: 493-503.
3Ott M, et al. Bioelectrical impedance analysis as a predictor of survival in patients with human immunodeficiency virus infection. Journal of Acquired Immune Deficiency Syndrome and Human Retrovirology 1995: 9:20-25.
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