What laboratory tests help determine if a patient (pt) is dehydrated?

Laboratory Tests for Determining Dehydration Status

Directly measured serum or plasma osmolality is the gold standard laboratory test for identifying dehydration, with a threshold of >300 mOsm/kg indicating dehydration. 1

Primary Laboratory Tests

• Serum/plasma osmolality is the most reliable laboratory indicator of hydration status, with values >300 mOsm/kg classified as dehydrated 1
• When direct osmolality measurement is unavailable, calculated serum osmolarity can be used with the formula: osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all measured in mmol/L), with an action threshold of >295 mmol/L 1
• Serum sodium, potassium, glucose, and urea are the key components that contribute to osmolality and should be measured together 1
• Blood urea nitrogen (BUN) is significantly related to dehydration severity and can help estimate fluid deficit 2
• Serum bicarbonate levels correlate with dehydration severity and enhance prediction accuracy when combined with urea measurements 2

Interpretation of Laboratory Values

• In low-intake dehydration, serum osmolality is elevated (>300 mOsm/kg) even when individual components (sodium, potassium, urea, glucose) may each remain within normal ranges 1
• When interpreting elevated serum osmolality, check that serum glucose and urea are within normal ranges; if not, these should be normalized by appropriate treatment 1
• Corrected serum sodium for hyperglycemia should be calculated (add 1.6 mEq to sodium value for each 100 mg/dL glucose >100 mg/dL) to accurately assess true sodium status 3, 4
• Elevated BUN with normal bicarbonate (≥15 mmol/L) has a high positive predictive value for moderate dehydration 2

Ineffective Laboratory Tests

• Urinary indices (color, specific gravity, osmolality) should NOT be used to assess hydration status in older adults as they have been shown to be unreliable 1, 5
• Bioelectrical impedance analysis (BIA) should NOT be used to assess hydration status as it has not been shown to be consistently diagnostic 1, 5
• Hematocrit and hemoglobin concentration alone are less sensitive at indicating moderate levels of dehydration compared to serum osmolality 6

Monitoring During Rehydration

• Electrolytes (sodium, potassium, chloride, bicarbonate) should be checked every 2-4 hours during initial treatment of severe dehydration 3, 7
• Blood glucose should be monitored every 1-2 hours until stable in cases of hyperglycemia-related dehydration 3, 4
• Calculate effective serum osmolality regularly to guide fluid management and prevent cerebral edema 3, 7

Clinical Application

• All older persons should be screened for dehydration when they contact the healthcare system, if clinical condition changes unexpectedly, and periodically when malnourished or at risk of malnutrition 1
• Laboratory assessment of hydration status should be performed rapidly in admitted patients, avoiding delays in the administration of initial therapy 1
• For patients with diabetes and suspected dehydration, obtain arterial blood gases, complete blood count, plasma glucose, BUN, electrolytes, and creatinine levels immediately 3, 4

Common Pitfalls

• Relying solely on clinical signs (skin turgor, mouth dryness, etc.) without laboratory confirmation can lead to misdiagnosis, as these signs are subtle and unreliable in adults, especially older adults 1, 8
• Confusing "dehydration" (loss of total body water) with "hypovolemia" (extracellular volume depletion) can lead to inappropriate treatment 8, 9
• Failing to correct serum sodium for hyperglycemia in diabetic patients can mask true sodium status 3, 4
• Not considering that dehydration may be multifactorial and rarely due to neglect, but rather results from a combination of physiological and disease processes 9