HYD: Assessing Hydration Status (2007)


 Oppliger RA, Magnes SA, Popowski LA, Gisolfi CV.  Accuracy od urine specific gravity and osmolality as indicators of hydration statua.  International J of Sport Nutrition and Exercise Metabolism 2005; 15: 236-251. 

Study Design:
Diagnostic, Validity or Reliability Study
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

2 parts to study:

1) To evaluate response of Usg and Uosm to a change in hydration status over the range from euhydration to 5% of weight loss by dehydration in controlled lab setting (compared to plasma osmolality). 

2) To evaluate the accuracy of Usg and Uosm assessments of hydration status in the field against criterion measure (Posm), using a medical decision model to evaluate the outcomes.

Inclusion Criteria:

Part 1)  physically fit, able to endure exercise in heat for sufficient duration to achieve wt loss of 5% body mass; informed consent

Part 2) Division I and III wrestling programs and nonwrestlers; no others mentioned

Exclusion Criteria:

1) Usg>1.015 were excluded.

2) none mentioned.

Description of Study Protocol:

Recruitment not mentioned



Part 1) Day before test day, subjects instructed to consume enough fluid to be fully-hydrated.  Upon awakening on test day, subjects required to drink 250 mL water before coming to lab.  Upon arrival to lab, Usg was measured to insure euhydration.  Body weight measured after voiding; rectal temp and heart rate assessed (monitored throughout); 10 mL blood sample collected for Posm.  Subjects entered environmental chamber (43C, 20% RH) and exercise don stationary bike or treadmill, dehydrating 1% total body mass in 30 min.  Blood and urine samples collected at 1% (30 min), 3% (1.5 hr), and 5% (2.5 hr) dehydration/loss of body  mass;  60 min recovery period, subjects consumed chilled water equivalent to amout of weight lost, 1/2 within first 30 min, the other 1/2 during final 30 min of recovery.  Sample collected at 30 and 60 min during recovery.

Part 2) Blood and urine sample collected in the field, various times of day; Usg assessed at time of measurement; U osm and Posm assayed later


Blinding used (if applicable)

 not mentioned

Intervention (if applicable)

 1) exercise to produce dehydration

Statistical Analysis

 Part 1) repeated measures ANOVA to test for significant differences between measurement at the 6 stages, sig level at p<0.05;  tests done for multiple comparisons when needed.

Part 2) Medical decision model: measuring sensitivity and specificity; ROC curve; Chi-square analyses, sig p< 0.05 ; power coefficient for Type II error using Cohen method; pearson correlation to comapare association between Usg and Uosm

Data Collection Summary:

Timing of Measurements

Part 1): 

Upon awakening on test day, subjects required to drink 250 mL water before coming to lab. 

Upon arrival to lab, Usg was measured; body weight; 

Rectal temp and heart rate assessed at baseline (monitored throughout);

10 mL blood sample collected for Posm. 

 Exercise period: Blood and urine samples collected at 1% (30 min), 3% (1.5 hr), and 5% (2.5 hr) dehydration/loss of body  mass; 

60 min recovery period: blood and urine Sample collected at 30 and 60 min during recovery.

Part 2:  Samples collected at various times of day (field setting)

Dependent Variables

  • Usg (refractometer)
  • Uosm (freezing point depression)
  • Posm (freezing point depression)


Independent Variables

 Hydration state

Control Variables

 1) Room temp

2) none

Description of Actual Data Sample:


Initial N:

Part 1: 12 M athletes

Part 2: 51 M:  47 wrestlers; 4 non-wrestlers

Attrition (final N):

Part 1) 8 M (final)


1) 20.9±1.8

2) no descriptive data on subjects

Ethnicity: not mentioned

Other relevant demographics: not mentioned

Anthropometrics 1) body mass: 78.6±4.0 kg; HR < 180 bpm; Rectal temp<39.5C; 2) not mentioned


1) Lab, university of Iowa

2) not mentioned


Summary of Results:





1% Loss 3% Loss 5% Loss 30 min Recovery

60 Min Recovery

Statistical Significance of Group Difference

Study I





Body weight  (kg) and loss (%)


78.6±4.0 kg


Kg not published


kg not published


74.6±3.8 kg

 % or kg not published

 %not published


 not mentioned



 1.018  1.020*  1.027*  1.033*



(sig diff from baseline)

Uosm  281.9  327.3  535.3*  605.3*  672.3*  597.0*  *p<0.006
Posm  287.6  290.8*  296.2*  304.0*  297.9*  291.3*  *p<0.006
Study II              
Usg  1.022±0.006            
Uosm  813.8±221.4            
Posm  292.6±5.6            
Correlation Usg & Uosm r=0.85           p<0.05


Other Findings

 Part 2):  Results of sensitivity/specificity, cutoff of Usg 1.020 to classify dehydration (Posm criterion for euhydration = 290 mosm/kg):

Low specificity: 31.3% of euhydrated subjects (true negatives, TN) correctly classified.  High sensitivity: 80% dehydrated subjects (true positives, TP) correctly classified.

Usg:  A stair-step efect is generated between TP and TN (Sensitivity and specificity) that reflects contrast between the sensitivity and specificity as the cut-off point for the test used in the medical decision model is increased.  Greatest number of subjects (N=33) was correctly classified as TP or TN at 1.015 (from X-square test, p=0.9) and 1.020 (p=0.2).

Uosm: cut-offs of 716 and 1050 mosm/kg; number of TN small ( n=3, 18.8%), thus high percentage classified as dehydrated (n=43); 93.8% euhydrated subjects correctly identified.  Greatest number of correctly classified (n=32) as TP or TN at 716 mosm/kg (from X-square test, p=0.2) and 800 mosm/kg (p=0.91).

ROC analysis: low power (did not exceed 0.35), no data points for Usg or Uosm significantly greater than equal likelihood (X-square test, p>0.05).


Author Conclusion:

Posm is the best method for assessing hydration status, and appears to be sensitive to small changes in hydration status, though hard to assess in a field setting.

Usg may be a viable method for assessing hydration status and changes in hydration status, and is able to be done in the field.  Cutoff of Usg>1.020 as marker for dehydration is suggested. 

Other factors, not measured in this study could adversely affect assessment of Usg and Uosm, and the accuracy of cut-offs as suggested by NATA and NCAA wrestling weight management program. 

Dynamic changes in Posm, Usg and Uosm during acute dehydration in a lab setting; in the field setting limited data from this study on how field measurements of Posm are related to Usg.


Funding Source:
Foundation associated with industry:
Reviewer Comments:

Usg is an accurate alternative to measuring Posm as an index of hydration status. 

*I completed the checklist taking both portions of the study into account, though had i completed a checklist for each part, Study 1 would have come out with a stronger rating than study 2. DMDV

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) Yes
  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? ???
  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? ???
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? No
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? ???
  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")? Yes
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? ???
  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.) Yes
  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? 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? Yes
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  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? 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? ???
  7.6. Were other factors accounted for (measured) that could affect outcomes? No
  7.7. Were the measurements conducted consistently across groups? No
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)? Yes
  8.6. Was clinical significance as well as statistical significance reported? ???
  8.7. If negative findings, was a power calculation reported to address type 2 error? Yes
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