How to Check for Dehydration
Measure serum osmolality directly, with a value >300 mOsm/kg confirming dehydration—this is the gold standard test and should replace reliance on unreliable clinical signs. 1, 2
Laboratory Assessment (Primary Method)
Gold Standard Test
- Direct serum osmolality measurement is the most accurate diagnostic test, with >300 mOsm/kg indicating dehydration in adults, particularly older adults 1, 2, 3
- This threshold is based on rigorous research and is associated with increased mortality and doubled risk of 4-year disability 1, 4
- The European Society for Clinical Nutrition and Metabolism gives this a Grade B recommendation with 94% consensus 4
Alternative When Direct Measurement Unavailable
- Calculate serum osmolarity using: 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L) 1, 2, 4
- Use an action threshold of >295 mmol/L for calculated osmolarity 1, 4
- Ensure glucose and urea are within normal range before interpreting, as abnormalities affect accuracy 2, 4
Supportive Laboratory Findings
- BUN to creatinine ratio >20:1 suggests dehydration, as urea is reabsorbed disproportionately during volume depletion while creatinine is not 2
- Both BUN and creatinine may rise, but BUN rises more dramatically due to increased reabsorption with antidiuretic hormone action 2
- In older adults with poor renal function, these renal parameters become unreliable 1, 2
Clinical Assessment (Adjunctive Only)
What NOT to Use
Do not rely on traditional clinical signs alone—they lack diagnostic accuracy, especially in older adults: 1, 4
- Skin turgor is unreliable (Grade A recommendation against use with strong consensus) 1, 4
- Mouth dryness is unreliable 1, 4
- Weight change alone is inadequate 1
- Urine color, urine specific gravity, and urine osmolality have inadequate diagnostic accuracy 4
- Bioelectrical impedance has not been shown to be diagnostically useful (Grade A recommendation against use with 100% consensus) 1, 4
Clinical Signs That May Support Diagnosis
When combined with laboratory values, these findings can provide supportive information: 5
- Thirst assessment using a 0-9 scale, with ≥4 indicating dehydration (sensitivity 100%, specificity 87% for intracellular dehydration and 71% for extracellular dehydration) 6
- Blood pressure assessment, including orthostatic changes 5, 7
- Eye findings such as sunken eyes (particularly in children) or reduced tear production 8, 5
- Mouth findings including mucous membrane wetness 5
Population-Specific Considerations
Older Adults
- All older persons should be considered at risk for low-intake dehydration 1
- Clinical judgment alone is highly fallible in this population 1
- Assessment of fluid intake by care staff is often highly inaccurate, with poor correlation (r = 0.122) between staff-recorded and actual intake, often underestimating by approximately 700 ml/day 1
- Request periodic serum osmolality assessment rather than relying on intake records 1
Children Under 2 Years
- Monitor for sunken eyes, which is a more reliable sign in pediatric populations 8
- Assess number of stools and ability to drink 8
- Very thirsty appearance warrants immediate reassessment 8
Practical Algorithm
First-line: Order direct serum osmolality measurement
If direct measurement unavailable: Calculate osmolarity using the formula
Check BUN:creatinine ratio as supportive evidence
20:1 supports dehydration diagnosis 2
Use thirst scale (0-9) as a practical screening tool
- ≥4 suggests dehydration 6
Do not delay laboratory confirmation based on clinical signs alone, as they are unreliable 1, 4, 5
Critical Pitfall to Avoid
The most common error is overreliance on clinical signs without laboratory confirmation. Studies show that systematic clinical assessment overdiagnoses dehydration compared to objective standards, though clinicians perform better at excluding it 7. The combination of history, physical examination, laboratory values, and clinical experience remains necessary, but laboratory confirmation with serum osmolality should drive definitive diagnosis 5, 3.