HYD: Assessing Hydration Status (2007)


Olde Rikkert MG, Deurenberg P, Jansen RW, van't Hof MA, Hoefnagels WH. Validation of multi-frequency bioelectrical impedance analysis in detecting changes in fluid balance of geriatric patients. J Am Geriatr Soc 1997 Nov;45(11):1345-51.

PubMed ID: 9361660
Study Design:
Cross-Sectional Study
C - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To determine the validity of Multi-Frequency Bioelectrical Impedance Analysis (MFBIA) in detecting clinically relevant changes of fluid balance in geriatric patients. To determine whether serial MFBIA correlates well with changes in weight and body fluid compartments in patients who are treated for fluid balance disturbances.  It also assesses whether MFBIA is sufficiently responsive to changes in fluid balance, compared with the within-subject variability in subjects with a stable fluid balance (to be useful in monitoring fluid balance).

Inclusion Criteria:
  • Subjects admitted to the Geriatric Medicine Center from 9/1/94 through 12/31/05
  • Subjects had to rate <1 on the Clinical Dementia Scale so they could be capable of giving written informed consent
Exclusion Criteria:
  • Patients with pacemakers
  • Terminally ill patients
  • Patients with psychogeriatric diseases likely to interfer with compliance and capacity to consent.
Description of Study Protocol:

Recruitment: From patients admitted to the Department of Geriatric Medicine of the University Hospital Nijmegen, The Netherlands.

218 total patients were screened for eligibility during the first days of admission. Written informed consent was obtained from the 78 eligible patients.


a) Fluid balance evaluation was clinically assessed twice each week by two physicians blinded from MFBIA results.  Assessments included standardized physical exam including evaluation of tongue and mucus membrane dryness, upper body muscle weakness, confusion, sunkenness of eyes and axillary moisture; weight measurement, lab tests including hematocrit, serum sodium, urea and creatinine levels, relevant data from medical history and documented nursing observations.  Patients were judged to be either underhydrated, overhydrated or euvolemic.  After monitoring the effects of therapy, each assessment was reconsidered.

b) Body composition analysis was performed twice in each patient using deuteriumoxide-and potassium bromide-dilution techniques to quantify relevant changes in total body water (TBW) and extracellular fluid (ECF).

c) MFBIA and weight measurements were performed daily in the morning, just after waking, while patients were still lying in bed.  Four-electrode MFBIA was performed on the nondominant side, and impedance was measured at four frequencies (1,5,50,100kHz)

Blinding (NA)

Intervention (NA)

Statistical Analysis: The usefulness of MFBIA as a discriminative instrument was analyzed by calculating the sensitivity of a single MFBIA judged by comparing it with the 95% reference range of MFBIA in euvolumic patients.  Significance of changes in weight, TBW, ECF, and impedance values were tested with paired t tests.  Pearson's correlation coefficients were calculated to relate changes in impedance to changes in body fluid compartments and weight. Guyatt's responsiveness index (RI) is used to determine how useful MFBIA is as an evaluative instrument for fluid balance. The RI of MFBIA was compared with the responsiveness for changes in body weight.

All episodes of dehydration and overhydration necessitated therapeutic adjustments. Therefore, the differences between the first observation and the mean of a subject's observations in the state of euvoluemia were considered to be clinically relevant changes. The within-subject variability was calculated by analysis of variance as the standard deviation of the observations during euvolemia, pooled over all patients.  The clinical relevant changes and within-subject standard deviations were calculated both absolutely and relatively to the euvolemic level as coefficients of variation.  RI's were calculated for individual changes in MFBIA and weight and overall within-subject standard deviation.  For the entire group, mean RI's in detecting dehydration and overhydration were calculated for MFBIA and weight.  Significance of the differences between RIs for the four frequencies and for daily weighing were tested by repeated measures analysis of variance.

Data Collection Summary:

Timing of Measurements: 14 of the subjects could not be studied until discharge because of a severely worsening clinical condition.  3 of these died shortly after the study ended.  However, data from all 53 subjects could be analyzed because they participated long enough in the study.  Mean participation was 29 days, ranging from 8-143 days.

Dependent Variables:  Changes in fluid balance, MFBIA values, changes in TCF and ECF

Independent Variables: Age, Gender, Presenting fluid status, ADL performance, mobility, morale and cognition


Description of Actual Data Sample:

Initial N: 53 Of the 78 eligible subjects, written informed consent was obtained form 53 adults, 17 males and 36 females

Attrition (final N):  Data from all 53 patients could be analyzed.  However, significant percentages of tests (planned fluid assessments, MFBIA and daily weighing) were unable to be performed 12.8-22.4% of the time for varying reasons.

Age: mean age 80 years, no other data given

Ethnicity: not specified

Other relevant demographics: 26 patients began study with abnormal fluid balance from various causes primarily inadequate water intake, vomiting, heart failure, overshooting dehydration treatment, and overshooting treatment for CHF.

Summary of Results:

In total, 1071 MFBIA measurements in 52 subjects were performed, 813 in euvolemia, 86 in dehydration, 101 in overhydration, and 71 measurements had to be excluded because fluid balance could not be assessed. Fluid balance was completely assessed 365 times in all 53 subjects. During this time 14 transitions from dehydration to euvolemia and 13 transitions from overhydration to euvolemia were monitored.  Rehydration of dehydrated patients caused an increase in TBW and ECF of 3.4±1.8L and 1.9±1.9L, respectively, which resulted in significant decreases in impedance of 1.33±67 at 1kHz and 93±61 at 100kHz (p=.001) Treatment of overhydrated patients caused a TBW and ECF loss of 3.8±4.2L and 3.1±3.8L, respectively, which resulted in significant increases in impedance of 104±72 at 1kHz and 81±68 at 100kHz (p<.001).   See Table One.

Sensitivity of a single MFBIA in diagnosing dehydration and overhydration was only 14% and 17%, respectively. 

Table 1:Patterns of Changes in Fluid Balance

Pattern Order N
A Euvolemia 27

Euvolemia to Dehydration 



Euvolemia to Overhydation 


D Dehydration to Euvolemia  9
E Overhydration to Euvolemia  8
F Dehydration to ±Overhydraion to Euvolemia  1
G Overhydration to Dyhyration to Euvolemia  1
H Overhydration  1

At all frequencies absolute changes in MFBIA correlated with changes in TBW and ECF caused by the diuretic treatment of overhydration.  (Table 2).

The increase of TBW, ECF or weight during treatment of dehydration did not correlate significantly with increases in MFBIA.

Table Two: Correlation of Changes (C) in MFBIA at 1, 5, 50 and 100kHz with Changes in TBW, ECF and Weight During Transisitions from Dehydration and OVerhydration to Euvolemia


Dehydration   P

Overhydration   P
r(Z1kHZ VS C TBW -0.06        .844 0.84            <.001
r(Z5kHZVS C TBW 0.11         .706 0.83            <.001
r(Z50kHZVS C TBW 0.20         .504 0.80              .002
r(Z100kHZVS C TBW 0.12         .677 0.80              .001
rZ1kHZ VS C ECF 0.16         .574 0.93            <.001
r(Z5kHZVS C ECF 0.29         .318 0.77              .002
r(Z50kHZVS C ECF 0.32         .264 0.73              .004
r(Z100kHZVS C ECF 0.33         .245 0.72              .005           
r(Z1kHZ VS C Wt 0.33         .247 0.77              .002
r(Z5kHZVS C Wt 0.41         .145 0.74              .004
r(Z50kHZVS C Wt 0.41         .145 0.71              .006
r(Z100kHZVS C Wt 0.45         .105 0.70              .007

 r = Pearson's coefficient of correlation.

At all frequencies absolute changes in MFBIA correlated with changes in TBW and ECF caused by the diuretic treatment of overhydration, but not during treatment of dehydration. 

The responsiveness index (RI's) for dehydration and overhydration were greater than 1, and there were no significant differences between RIs at the four frequencies.

The RI of weight change in detecting dehydration and overhydration did not differ significantly from the RI's of MFBIA.

Table Three: Mean Differences (C) Between Initial Total Body Impedance (Z) During Dehydration and Overhydration Episodes and the Responsiveness Indexes (RI) of MFBIA for These Disturbances of Fluid Balance



C dehyd-euv, n=14


P Coverhydr-euv, n=13 P RIdehydr-euv,n=14 RIoverhyd-euv,n=13
Z1kHz  133±67  <.001  -104±72  <.001  2.6±1.8  2.0±1.3
Z5kHz  123±75  <.001  -  97±71  <.001  2.6±2.3  2.1±1.4
Z50kHz  113±75    .003  -  85±70    .001  2.9±2.6  2.1±1.7
Z1001kHz   93±63  <.001  -  81±68    .001  2.4±2.1  2.1±1.7


Author Conclusion:


Overall, the data does not support the suggestion that MFBIA is superior to the conventional single frequency (50kHz) impedance analyzers in monitoring fluid balance in geriatric patients. Single impedance measurement proved to be of little help in the diagnosis of dehydration and overhydration. MFBIA cannot accurately quantify the magnitudes of fluid volumes.  Moreover, no single frequency was superior in tracking changes in fluid balance over time.  Single frequency impedance analysis can be a valuable instrument in monitoring fluid balance because it is sensitive to clinically relevant changes, within subject variability in euvolemia is low, and the measurements are easy to perform.

 Limitations of the study according to the authors include:

  • Only 25% of the total number of patients admitted during the study period could be studied secondary to degree of dementia.  However, since dementia is not likely related to the variables measured in the study, the external validity to the study was probably not jeopardized.

Body composition analysis was not completely independent of the clinical assessments. Changes in body composition were caused by the treatment of fluid imbalance.

Funding Source:
Government: Nestor, Ministry of Education Science and Culture, Ministry of Health Welfare and Sports
Reviewer Comments:

This was a study aimed at the validation of diagnostic measures. Though it did not end up proving the effectiveness of single impedance measurements in detecting dehydration and overhydration, serial impedance measurements were demonstrated to be a valid non-invasive technique used in clinical practice to improve monitoring fluid balance in geriatric patients.

Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) N/A
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? Yes
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? Yes
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? Yes
4. Was method of handling withdrawals described? N/A
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? Yes
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? Yes
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? N/A
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? Yes
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? N/A
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? N/A
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? N/A
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? No
9. Are conclusions supported by results with biases and limitations taken into consideration? N/A
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes