What laboratory tests indicate dehydration?

Laboratory Tests Indicating Dehydration

Serum/plasma osmolality is the single most reliable laboratory test for diagnosing dehydration, with values >300 mOsm/kg definitively indicating dehydration. 1, 2

Primary Diagnostic Tests

Serum Osmolality (Gold Standard)

• Direct measurement of serum/plasma osmolality is the gold standard for determining dehydration status. 1, 2
• Values >300 mOsm/kg classify a patient as dehydrated 1
• This test reliably detects water-loss (intracellular) dehydration even when individual electrolytes remain within normal ranges 1
• Plasma osmolality represents the most valuable, objective surrogate marker of hypertonic dehydration, though it remains underutilized in clinical practice 2

Calculated Osmolarity (When Direct Measurement Unavailable)

• Use the formula: osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L) 1
• Action threshold is >295 mmol/L 1
• This calculation provides a reasonable alternative when direct osmolality measurement is not immediately available 1

Essential Supporting Laboratory Tests

Order these tests together to fully assess dehydration:

• Serum sodium (corrected for hyperglycemia if glucose is elevated) 3, 1
• Serum potassium 3, 1
• Blood glucose 3, 1
• Blood urea nitrogen (BUN) 3, 1
• Serum creatinine 3, 1
• Electrolyte panel (including chloride and bicarbonate) 3

BUN/Creatinine Ratio

• Elevated BUN with normal or mildly elevated creatinine suggests dehydration 4
• In dehydration without central diabetes insipidus, serum urea is typically elevated (mean 15.4 mmol/L) due to ADH-mediated renal reabsorption 5
• This contrasts with central diabetes insipidus where urea may actually decrease during dehydration (mean 2.9 mmol/L) 5

Critical Interpretation Guidelines

Correcting Sodium for Hyperglycemia

For each 100 mg/dL glucose above normal, add 1.6 mEq to the measured sodium value to obtain the corrected serum sodium. 3, 1

• This correction is essential in diabetic patients to avoid masking true sodium status 1
• Failure to correct sodium for hyperglycemia is a common pitfall that leads to misdiagnosis 1

Osmolality Interpretation Caveats

• Before attributing elevated osmolality solely to dehydration, verify that serum glucose and urea are within normal ranges 1
• If glucose or urea are abnormal, these should be normalized by appropriate treatment first 1
• In low-intake dehydration, osmolality may be elevated (>300 mOsm/kg) even when individual components remain within normal ranges 1

Monitoring During Treatment

Frequency of Laboratory Monitoring

• Check electrolytes (sodium, potassium, chloride, bicarbonate) every 2-4 hours during initial treatment of severe dehydration 3, 1
• Monitor blood glucose every 1-2 hours until stable in hyperglycemia-related dehydration 1
• Calculate effective serum osmolality regularly to guide fluid management and prevent cerebral edema 1
• Obtain BUN, creatinine, and osmolality every 2-4 hours during acute management 3

Rate of Correction

• Do not allow osmolality changes to exceed 3 mOsm/kg/hour to prevent cerebral edema 6
• This is particularly critical in severe hypernatremic dehydration 6

Tests That Should NOT Be Used

Unreliable Urinary Markers

• Urinary indices (color, specific gravity, osmolality) should NOT be used to assess hydration status in older adults 1
• These markers have been shown to be unreliable in this population 1
• While urine concentration occurs in dehydration (pH ~5.0), this is not a reliable diagnostic marker 6

Ineffective Technology

• Bioelectrical impedance analysis (BIA) should NOT be used to assess hydration status 1
• BIA has not been shown to be consistently diagnostic 1

Initial Laboratory Workup Algorithm

For any patient with suspected dehydration, immediately obtain: 3, 1

1. Serum/plasma osmolality (direct measurement preferred)
2. Complete metabolic panel (sodium, potassium, chloride, bicarbonate, BUN, creatinine, glucose)
3. Complete blood count with differential
4. Urinalysis (for infection screening, not hydration assessment)
5. Arterial or venous blood gas (if acidosis suspected)
6. Electrocardiogram (to assess for electrolyte-related cardiac effects)

In diabetic patients with suspected dehydration, additionally obtain: 3

• Serum ketones
• Calculated anion gap
• Venous pH (adequate for monitoring; repeat arterial blood gases generally unnecessary)

Common Diagnostic Pitfalls

• Relying solely on clinical signs (skin turgor, dry mouth) without laboratory confirmation leads to misdiagnosis 1, 7
• Clinical signs are subtle and unreliable in adults, especially older adults 1, 2
• Clinical assessment overdiagnoses dehydration (70% sensitivity, 69% specificity) but performs better at excluding it (90% negative predictive value) 7
• Individual clinical symptoms or signs have good specificity (65-90%) but poor sensitivity (<55%) 7
• The best diagnostic approach combines history, physical examination, laboratory values, and clinical experience rather than relying on any single test 4, 2