interpreting test results
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effects of hydration on fat-free mass

The following information may be useful when interpreting a patient's hydration status or total body water (TBW) test results.

The analyzer provides information on TBW expressed as liters of water, water as a percentage of body weight, water as a percentage of fat-free mass (or lean body mass), intracellular water, and extracellular water.

Interpreting Fat Mass Test Results
Hydration status can be useful when reviewing the results of a body composition test. For an adult, body water is expected to be 69-74% of fat-free mass. For an adolescent, it is expected to be slightly higher. A reading below 69% may indicate dehydration and result in a fat mass reading that is 3-5% too high. Your professional judgment is needed in these cases to confirm that the patient is dehydrated. (See Other Factors below.)

A low reading (68% for example) indicates statistical dehydration in an individual when compared to the normal population as a group, but may be the normal hydration status for this individual and should not automatically be a source of concern. In the normal population, the theoretical hydration coefficient of fat-free mass is usually assumed to be 0.723, or 72.3%. This represents the ratio of TBW to fat-free mass (FFM), (Pace and Rathburn, Journal of Biological Chemistry, 158). In normal men, the actual ratio is as low as .65 and as high as .85 in cases of severe anasarca (temporary fluid imbalance - Sheng and Huggins). In chronically ill persons values as high as .80 have been observed (Moore and Boyden).

Other Factors
The analyzer's TBW reading gives you an initial indication of dehydration but factors such as the use of diuretics, anabolic medications, diet, hydration habits, and exercise habits should be considered when evaluating an individual's hydration status. If, after considering all these factors you conclude that the patient is dehydrated, you may consider adjusting the fat mass readings.

The following guidelines are recommended for adults:

Total Body Water
(% Lean Weight)
(% Fat)

69% and above none
68.0-68.9% subtract 1%
67.0-67.9% subtract 2%
66.0-66.9% subtract 3%
below 66.0% subtract 4%

No adjustments are recommended for adolescents.

Whenever a weight management program is indicated we recommend that patients drink at least 4 eight ounce glasses of water per day.

Adjusting Fat Mass Results
If, after running a test you choose to adjust fat mass downward to compensate for patient dehydration:

1. Press the DATA key and verify correct data (Age, Ht, etc.) or re-enter correct data.
2. Note the resistance and reactance readings from the previous (dehydrated) printout.
3. Note the percent fat mass from the previous (dehydrated) printout.
4. Subtract the adjustment factor from the table above from the fat mass identified in step (3).
5. Note this adjusted percent fat mass value.
6. Press the TEST key followed by the ENTER key. Enter a resistance value that is lower than the resistance value noted in Step (2). Enter the same reactance value as noted in Step (2). Press the TEST or ENTER key.
7. Review the new results. Compare these results to the adjusted fat mass value.
8. Repeat until the new results match the adjusted percent fat mass values from step (5). Press the PRINT key.
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