What laboratory tests indicate dehydration?

Laboratory Tests Indicating Dehydration

Serum or plasma osmolality >300 mOsm/kg is the single most reliable laboratory test for diagnosing dehydration and should be directly measured whenever possible. 1, 2

Primary Laboratory Tests

Serum/Plasma Osmolality (Gold Standard)

• Direct measurement of serum osmolality is the most evidence-based approach with Grade B recommendation and 94% expert consensus from the European Society for Clinical Nutrition and Metabolism 1, 2
• Values >300 mOsm/kg definitively indicate low-intake dehydration and require immediate intervention 1, 2
• This threshold is based on rigorous research separating hydrated from dehydrated states and is associated with increased mortality and doubled risk of 4-year disability 1
• Direct measurement is superior to calculation because it captures the true physiological state 3

Calculated Serum Osmolarity (When Direct Measurement Unavailable)

• Use this formula: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all values in mmol/L) 1, 2
• Action threshold is >295 mmol/L when using the calculated formula 1, 2
• This receives Grade B recommendation with 94% agreement as a second-line option 1
• For hyperglycemic crises specifically, use the simplified formula: 2[measured Na (mEq/L)] + glucose (mg/dL)/18 4, 1

Blood Urea Nitrogen (BUN)

• Elevated BUN is a strong indicator of dehydration, particularly when combined with other markers 4, 2
• In dehydrated children, BUN (urea) concentration shows significant correlation with dehydration severity (P < 0.00001) 5
• BUN ≥16.7 mmol/L (≥100 mg/dL) predicts moderate to severe dehydration 5
• BUN <6.7 mmol/L (<40 mg/dL) is associated with only mild to moderate dehydration 5
• Critical caveat: In central diabetes insipidus, BUN paradoxically decreases during dehydration (mean 2.9 mmol/L) due to lack of ADH-mediated urea reabsorption, while other causes of dehydration show elevated BUN (mean 15.4 mmol/L) 6

Serum Bicarbonate

• Bicarbonate levels correlate with dehydration severity (P = 0.01) 5
• Combining BUN with bicarbonate enhances diagnostic accuracy: when BUN is normal (<6.7 mmol/L) and bicarbonate ≥15 mmol/L, there is 93% positive predictive value for mild dehydration 5
• When BUN is high (≥16.7 mmol/L) and bicarbonate ≥15 mmol/L, moderate dehydration can be predicted with 100% positive predictive value 5
• Low bicarbonate (<15 mmol/L) is found in 60% of mildly dehydrated patients, but only 7% with bicarbonate ≥15 mmol/L have severe dehydration 5

Serum Sodium (With Important Corrections)

• Serum sodium must be corrected for hyperglycemia: add 1.6 mEq to the sodium value for each 100 mg/dL glucose above 100 mg/dL 4, 1, 2
• Uncorrected sodium can mask true sodium status in diabetic patients 2
• Sodium alone does NOT correlate with dehydration severity and should not be used in isolation 5
• Sodium is a component of osmolality calculation but requires interpretation in context 1, 2

Serum Creatinine

• Should be obtained as part of the initial chemistry profile 4, 2
• Helps assess renal function and distinguish prerenal azotemia from intrinsic renal disease 4
• Monitor every 2-4 hours during active treatment of severe dehydration 2

Complete Initial Laboratory Panel

For suspected dehydration, obtain immediately: 4, 2

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

Monitoring During Treatment

• Check electrolytes, glucose, BUN, creatinine, and osmolality every 2-4 hours during initial treatment of severe dehydration 4, 2
• Blood glucose should be monitored every 1-2 hours until stable in hyperglycemia-related dehydration 2
• The induced change in serum osmolality should not exceed 3 mOsm/kg/hour to prevent cerebral edema 1, 2
• Venous pH can substitute for arterial blood gases in most cases (venous pH is typically 0.03 units lower) 4

Tests That Should NOT Be Used

Grade A recommendations AGAINST the following: 1, 2

• Urine specific gravity, urine color, and urine osmolality have inadequate diagnostic accuracy per Cochrane systematic review 1
• Bioelectrical impedance analysis has not been shown to be consistently diagnostic (100% consensus against use) 1, 2
• Clinical signs alone (skin turgor, mouth dryness, weight change) are unreliable and should not be used without laboratory confirmation 1, 2, 7

Critical Pitfalls to Avoid

• Never rely on clinical signs without laboratory confirmation, especially in older adults where signs are subtle and unreliable 1, 2, 7
• Always correct sodium for hyperglycemia in diabetic patients to avoid missing true hyponatremia 4, 2
• Ensure glucose and urea are within normal ranges when interpreting osmolality, as abnormalities in these independently elevate values 1, 2
• Do not use urinary indices as they are unreliable in adults 1, 2
• Consider central diabetes insipidus if BUN is paradoxically low despite clear dehydration (sodium >155 mmol/L with BUN <3 mmol/L) 6

Specific Clinical Contexts

Hyperglycemic Crises (DKA/HHS)

• Effective serum osmolality ≥320 mOsm/kg defines Hyperosmolar Hyperglycemic State 1
• DKA typically presents with osmolality <320 mOsm/kg 1
• Use corrected sodium and effective osmolality formula for accurate assessment 4, 1

Older Adults

• All older persons should be screened for dehydration when contacting healthcare, with clinical condition changes, or when malnourished 2
• Serum osmolality >300 mOsm/kg indicates low-intake dehydration requiring immediate intervention 1, 2
• Clinical signs are particularly unreliable in this population 1, 2