What are the most important lab tests to assess hydration status in an elderly patient?

Laboratory Tests for Hydration Status in an 80-Year-Old

The four most important laboratory tests, rank-ordered, are: (1) directly measured serum or plasma osmolality, (2) calculated serum osmolarity using the electrolyte equation if direct measurement is unavailable, (3) serum sodium concentration, and (4) serum glucose and urea to interpret osmolality correctly. 1

1. Directly Measured Serum or Plasma Osmolality (First Priority)

This is the gold standard and primary indicator of hydration status in older adults. 1

• Serum osmolality >300 mOsm/kg indicates low-intake dehydration and should trigger immediate intervention 1
• 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
• The ESPEN guidelines give this a Grade B recommendation with 94% consensus, making it the most evidence-based approach 1
• Direct measurement is superior to calculation because it captures all osmotically active particles 1, 2

Why This Test Works in Older Adults

• When fluid intake is insufficient, intracellular and extracellular fluid becomes concentrated, raising serum osmolality 1
• Unlike younger adults, older patients have impaired renal function, making renal parameters unreliable 1
• Clinical judgment alone is highly fallible in this population 1

2. Calculated Serum Osmolarity (Second Priority - If Direct Measurement Unavailable)

Use the equation: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L) 1

• Action threshold is >295 mmol/L (note: lower than the 300 mOsm/kg threshold for direct measurement) 1
• This receives a Grade B recommendation with 94% agreement from ESPEN 1
• The calculation requires obtaining sodium, potassium, glucose, and urea levels 1, 2

Critical Caveat

• This is explicitly a second-line option when direct osmolality measurement is not available 1
• The different thresholds (295 vs 300) reflect the calculation's limitations 1

3. Serum Sodium Concentration (Third Priority)

Sodium is the primary electrolyte contributing to serum osmolality and has independent diagnostic value. 2, 3

• Elevated sodium concentration indicates hyperosmolar dehydration 3
• Sodium, along with chloride, bicarbonate, and potassium, comprises the major osmotically active components 2
• This test is readily available and helps interpret osmolality results 2

Important Context

• Serum sodium must be interpreted alongside osmolality, not in isolation 2
• Hypernatremia reflects the concentrated state characteristic of low-intake dehydration 4

4. Serum Glucose and Urea (Fourth Priority - For Interpretation)

These tests are essential for correctly interpreting elevated osmolality. 1, 4, 2

• Elevated glucose or urea can independently raise osmolality without true dehydration 1, 4
• Interpretation of raised serum osmolality (>300 mOsm/kg) as dehydration depends on these values being within normal range 1, 4
• Both are included in the calculated osmolarity equation 1

Clinical Pitfall to Avoid

• Never diagnose dehydration based on elevated osmolality alone without checking glucose and urea 4, 2
• Hyperglycemia in uncontrolled diabetes or elevated urea in renal failure can create false-positive results 4

Tests That Should NOT Be Used

The following tests have been proven unreliable and should be explicitly avoided in 80-year-olds: 1

Urinary Tests (Grade A Recommendation Against Use)

• Urine specific gravity, urine color, and urine osmolality have inadequate diagnostic accuracy 1, 5, 6
• A Cochrane systematic review found none were consistently useful 1, 7
• The largest study to date (313 participants) confirmed urinary measures should not be used 6
• ESPEN gives a Grade A recommendation with 83% consensus that these SHALL NOT be used 1

Clinical Signs (Grade A Recommendation Against Use)

• Skin turgor, mouth dryness, weight change are unreliable 1, 3
• These signs have been shown not to be usefully diagnostic in multiple studies 1, 7

Bioelectrical Impedance (Grade A Recommendation Against Use)

• Has not been shown to be usefully diagnostic 1
• ESPEN gives this a Grade A recommendation with 100% consensus against use 1

Practical Implementation Algorithm

1. Order directly measured serum osmolality as first-line test 1
2. If direct measurement unavailable, order: sodium, potassium, glucose, and urea to calculate osmolarity 1
3. Always check glucose and urea alongside osmolality to rule out confounding causes 1, 4, 2
4. Interpret results:
   • Direct osmolality >300 mOsm/kg = dehydration 1
   • Calculated osmolarity >295 mmol/L = dehydration 1
5. Reassess regularly until corrected, then monitor periodically 1