HYD: Assessing Hydration Status (2007)


 Popowski LA, Oppliger RA, Patrick Lambert G, Johnson RF, Kim Johnson A, Gisolf CV. Blood and urinary measures of hydration status during progressive acute dehydration. Med Sci Sports Exerc. 2001, May;33(5):747-53

PubMed ID: 11323543
Study Design:
Diagnostic, Validity or Reliability Study
C - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To determine whether:

  • Plasma osmolarity (Posm) is sensitive to small incremental changes in hydration status
  • Urine specific gravity (Usg) can accurately identify a state of euhydration
  • Usg and urine osmolarity (Uosm) are sensitive indicators of changes in hydration status
  • Usg and Uosm correlate with Posm

Additionally, we sought to determine if reagent strips accurately measure Usg compared with refractometry measurements.

Inclusion Criteria:
Physically fit male athletes, able to endure exercise in the heat to achieve a body weight loss equal to 5% of their total body weight
Exclusion Criteria:
None reported
Description of Study Protocol:


 Not described


 Study of sensitivity and specificity of a diagnostic test (prospective design)

Blinding used (if applicable)


Intervention (if applicable)

Subjects dressed in layered clothing to promote fluid loss via perspiration and exercised in an environmental chamber maintained at 40 degrees C, 20% relative humidity

  • Dehydration protocol: Exercise on a stationary cycle and/or treadmill enabled subjects to progressively lose 1%, 3%, and 5% of their body weight via dehydration.
  • Rehydration protocol: Within the first 30 min of recovery, subjects consumed water equal to one-half of their body weight loss. From 30 to 60 min of recovery, subjects consumed a second volume of water to replace 100% of their body weight loss.

Statistical Analysis

  • One-way, repeated-measures ANOVA. When significatn main effects were observed, eight pairwise follow-up comparisons were made using a Scheffe F-test adjusted for the number of follow-up tests performed. P<0.006 was used for the significance level.
  • Reported correlation coefficients were calculated by pooling correlations for each subjects as described in Snedecor and Cochran.
Data Collection Summary:

Timing of Measurements

At baseline, 1%, 3%, and 5% dehydration, and at 30 and 60 min of recovery.

Dependent Variables

At each of these stages, a urine sample was also collected to measure Usg and Uosm.

  • Urine specific gravity (Usg): Usg was determine dby: a) Atago optical Usg refractometer calibrated with deionized water and b) N-Multistix SG reagent strips (Bayer Corporation, Elkhart, IN)

Independent Variables

  • Plasma osmolarity (Posm): A catheter inserted into an antecubital vein provided blood samples to measure Posm at baseline, 1%, 3%, and 5% dehydration, and at 30 and 60 min of recovery. Posm was determined via freezing point depression with an osmometer.
  • Urine osmolarity (Uosm): Uosm was determined via freezing point depression with an osmometer.

False positive and salse negative tests, comparing Usg with Posm

  • False positive: a person who was euhydrated based on the measurement of Usg (<=1.020) but dehydrated based on the measurement of Posm (>290 mOsm.L-1)
  • False negative: a person who was dehydrated based on the measurement of Usg (>1.020) but euhydrated based on the measuremtn of Posm (<290 mOsm.L-1)

Control Variables


Description of Actual Data Sample:


Initial N: 12

Attrition (final N): 12

Age: 22.8

Ethnicity: No data

Anthropometrics: Weight = 73.4 kg, Height = 177.6 cm; % Body fat = 8.0

Location: US


Summary of Results:
I. % Body weight loss
  • Subjects dehydrated to achieved an average body wieght loss of 1.18 +- 0.17%, 3.13 +- 0.16%, and 5.17 +- 0.17% of total body weight, at each successive stage of dehydration, respectively. An average of 39 min was required to reach a state of 1% dehydration, an additional 58 min to reach 3% dehydration, and an additional 62 min to reach 5% dehydration.
  • They lost an average of 3.88 +- 0.64 kg of body weight to achieve 5% dehydration in 168 +- 50 min.
II. Blood and urine analysis



1% BW loss

3% BW loss

5% BW loss

30 min recovery

60 min recovery

Posm (mOsm.L-1)







Uosm (mOsm.L-1)














 *Significant difference from baseline euhydration condition (p<0.05)

III. Correlation between tests

  • Measurements of Usg using N-Multistix SG reagent strips correlated well with measurements by regractometry (r=0.68, P<0.02)
  • Usg and Uosm correlated well (r=0.82, P<0.01)
  • Uosm correlated moderately with Posm (r=0.43, P>0.10)

IV. False positive and false negative tests, comparing Usg with Posm 

Usg (standard)

Posm (mOsm.L-1)

  Baseline 1% BW loss 3% BW loss 5% BW loss





False positive

False negative













False positive

False negative









Author Conclusion:

Plasma osmolarity (Posm) accurately identifies a state of euhydration and is sensitive to changes in hydration status during acute dehydration and rehydration.

Urine specific gravity and urine osmolarity are also sensitive to changes in hydration status but lag behind during periods of rapid body fluid turnover and therefore correlate only moderately with plasma osmolarity during acute dehydration.

Funding Source:
Foundation associated with industry:
Reviewer Comments:

Although authors concluded that Posm is the best measure of euhydration and accurately reflect modest changes in hydration status during acute dehydration achieved by exercise in a warm environment, the analysis of "false positive" and "false negative" test used Usg as the reference standard. This is not consistent with commonly recognized definitions for "false positive" and "false negative".

Limitation: male athletes only

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? N/A
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? N/A
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) N/A
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? N/A
  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? N/A
  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? N/A
  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")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  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? Yes
5. Was blinding used to prevent introduction of bias? ???
  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.) ???
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  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? ???
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? N/A
  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? N/A
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? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  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? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  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