What labs should be drawn to rule out dehydration?

Laboratory Tests to Rule Out Dehydration

Directly measured serum or plasma osmolality is the single most important laboratory test to rule out dehydration, with a threshold >300 mOsm/kg definitively indicating low-intake dehydration. 1, 2

Primary Laboratory Test

• Serum osmolality (directly measured) is the gold standard with Grade B recommendation and 94% expert consensus from the European Society for Clinical Nutrition and Metabolism 1, 2
• Values >300 mOsm/kg confirm dehydration and require immediate intervention 1, 2
• This threshold is based on rigorous research that separated hydrated from dehydrated states in controlled studies and is associated with increased mortality risk and doubled 4-year disability risk in older adults 1, 2
• Normal serum osmolality ranges from 275-295 mOsm/kg 2, 3

Alternative When Direct Measurement Unavailable

• Use calculated osmolarity when direct measurement is not available: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all values in mmol/L) 1, 2
• Action threshold for calculated osmolarity is >295 mmol/L 1, 2
• This also carries Grade B recommendation with 94% agreement 1, 3

Complete Initial Laboratory Panel

When dehydration is suspected, obtain immediately: 2

• Serum osmolality (direct measurement preferred)
• Complete metabolic panel (includes sodium, potassium, chloride, glucose, BUN, creatinine)
• Arterial or venous blood gas
• Complete blood count with differential
• Urinalysis

Elevated BUN is a strong supporting indicator of dehydration, particularly when combined with osmolality measurement. 2

Critical Interpretation Caveats

• Always ensure glucose and urea are within normal ranges when interpreting osmolality, as abnormalities in these independently elevate values and can confound the diagnosis 1, 2, 3
• Correct sodium for hyperglycemia in diabetic patients: for each 100 mg/dL glucose above 100 mg/dL, add 1.6 mEq to the sodium value to avoid missing true hyponatremia 2, 3
• Serum osmolality reflects intracellular water loss (low-intake dehydration), while extracellular water loss from diarrhea, vomiting, or renal sodium loss presents with normal or low plasma osmolality 1

Tests That Should NOT Be Used

The following have been definitively shown to lack diagnostic accuracy and carry Grade A recommendations AGAINST their use: 1, 2

• Urine specific gravity - inadequate diagnostic accuracy per Cochrane systematic review 1, 2, 3
• Urine color - not consistently useful 1, 2, 3
• Urine osmolality - unreliable in adults 2, 4
• Skin turgor - not usefully diagnostic in older adults 1
• Mouth dryness - not consistently useful 1
• Weight change - not reliably diagnostic 1
• Bioelectrical impedance - Grade A recommendation against use with 100% consensus 1, 2

Research confirms that urine concentration thresholds classified 27-55% of athletes as dehydrated while none met clinical dehydration criteria by blood markers, demonstrating that urinary indices reflect kidney response rather than actual hydration status 4

Monitoring During Treatment

For patients with confirmed dehydration (osmolality >300 mOsm/kg): 2, 3

• Check electrolytes, glucose, BUN, creatinine, and osmolality every 2-4 hours during initial treatment of severe dehydration
• Monitor blood glucose every 1-2 hours until stable in hyperglycemia-related dehydration
• The induced change in serum osmolality should not exceed 3 mOsm/kg/hour to prevent cerebral edema

Special Population Considerations

All older adults should be screened for dehydration when contacting the healthcare system, with clinical condition changes, or when malnourished. 1, 2

• In older adults, renal function is often impaired, making renal parameters less reliable indicators 1, 5
• Clinical judgment is highly fallible in older adults and should never be used without laboratory confirmation 1
• Raised serum osmolality >300 mOsm/kg in older adults is consistently associated with increased mortality in high-quality cohort studies 1

Common Pitfalls to Avoid

• Never rely on clinical signs alone without laboratory confirmation, especially in older adults where signs are subtle and unreliable 1, 2
• Do not use oral rehydration therapy or sports drinks for low-intake dehydration - these are indicated for volume depletion from diarrhea/vomiting, not for hyperosmolar dehydration 1
• Do not confuse intracellular dehydration (high osmolality) with extracellular volume depletion (normal/low osmolality) - they require different treatments 1