Notes On Phase Angle Interpretation

Phase angle is an actual measure based on resistance and reactance measures in the body. How the body resists and reacts to the small amount of electricity it takes to get around and/or through all the cells is recorded in these two measures. Resistance and reactance numbers are “plotted” to meet at the point of the frequency of the particular BIA machine (in this case it is 50 kHz). That determines the phase angle (see Figure 4).

Figure 4. Illustration of Phase Angle Alpha

phase angle = arctangent of (reactance/resistance)*(180/pi)

Study authors suggest that a higher phase angle is related to better health. Or, more accurately, a low phase angle (below 4.8) is related to morbidity and mortality. So, it follows that keeping phase angle high (or even raising it) is a good thing. But, keep reading to compare estimated body compartment measures to phase angle for value in evaluating an individual before you make that decision.

Body cell mass, on the other hand, is not a direct measure. BCM is calculated using the resistance and reactance measures along with other factors such as weight, height, and gender. So, the number we get for BCM is only as good as the equations we have. At this point, we think they are pretty good… but that will change as greater numbers and types of patients are tested with both BIA and the gold standards used to validate the equations (such as DXA or TBK+) and technology is further refined.

So one could argue that you should go with a direct measure and not rely on derived calculations (even if they are fairly well validated). And, if you are studying a large enough population, this “purist” stance makes sense. But, we are trying to use the technology to clinically benefit individual patients. So, we need to put it into terms that a clinician and patient can use to determine appropriate therapy or recommendations.

The other difficulty with using phase angle to determine the well-being of individual clients is that it appears to have very specific relationships with the calculated amounts of BCM, extracellular mass (ECM or ECT), and fat. The relationships are shown below:

Positive relationship (when this rises, phase angle rises): BCM

Inverse relationship (when these rise, phase angle falls): ECT, fat

Thus, interpreting phase angle needs to take into consideration why it may be low or high. To recap reality: you can make phase angle go up if you increase BCM… and that is good, right? But, you can also make phase angle go up if you get dehydrated… that can’t be good. And, you can make phase angle go up if you lose fat… that can be both good and bad depending on how much you have originally.

Let’s look at the other side: phase angle goes down when you lose BCM or when you gain ECM or fat. Well, losing BCM has never been reported as a good thing. And, if you gain fluid to raise ECM because of infection, that is bad. If you gain fat (as is sometimes done to excess in patients who do not respond to nutritional rehabilitation normally), then phase angle drops. But, phase angle will also drop if you re-hydrate a dehydrated person – making the drop a good thing. And if you need to gain fat to a normal level and are able to do so, phase angle will drop for that, too! This means that the best phase angles are in people with tons of muscle, extremely dehydrated, and almost no fat to sustain life processes.

Is raising phase angle always good? Is lowering it always bad? “No” is the answer to both of those. One thing that seems to be common is that if phase angle is below about 4.8, some of the body compartment volumes are probably out of whack. What it really means is that just because the phase angle looks good, it doesn’t mean the person is doing well and needs no intervention. And if the phase angle drops, it doesn’t necessarily mean the person is in worse shape.

A clinician would still have to look at the body compartments to figure out which direction is most appropriate. So we are back to the idea that in an individual patient we still have to look at the calculated BCM level (along with ECM and fat levels) to decide on and monitor therapies that will provide clinical benefit. And if we do the right thing with the calculated body compartments, phase angle will hold its own.

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