Should corrected sodium or measured sodium be used for osmolality calculation?

Use Measured Sodium for Osmolality Calculation

For calculating effective serum osmolality, use the measured (uncorrected) sodium value, not the corrected sodium. 1

The Critical Distinction

The American Diabetes Association guidelines explicitly separate these two calculations for different clinical purposes:

• Effective serum osmolality calculation: 2[measured Na (mEq/L)] + glucose (mg/dL)/18 1
• Corrected sodium: Used separately to guide fluid choice, calculated as measured Na + 1.6 × ([glucose - 100]/100) 1, 2

Why This Matters Clinically

Measured sodium reflects the actual osmotic gradient that drives water shifts between compartments and determines true serum tonicity. 3 The corrected sodium is a mathematical construct that estimates what the sodium would be if glucose were normal—it's useful for fluid selection but does not represent the actual osmotic state. 2

The Two-Step Approach:

1. Calculate osmolality using measured sodium to assess severity and monitor treatment (e.g., diagnostic criterion for HHS is effective osmolality >320 mOsm/kg) 1

2. Calculate corrected sodium separately to determine fluid type:

   • Normal/high corrected sodium → use 0.45% NaCl 2
   • Low corrected sodium → use 0.9% NaCl 2

Common Pitfall to Avoid

Do not use corrected sodium in the osmolality formula. This would artificially lower the calculated osmolality and misrepresent the patient's true hyperosmolar state, potentially leading to overly aggressive fluid resuscitation. 1 The goal is to decrease osmolality by no more than 3 mOsm/kg/h to prevent cerebral edema. 1, 2

Practical Example

For a patient with measured Na 145 mEq/L and glucose 900 mg/dL:

• Effective osmolality = 2(145) + 900/18 = 340 mOsm/kg 1
• Corrected sodium = 145 + 1.6 × (800/100) = 157.8 mEq/L 1, 2

Use the 340 mOsm/kg to confirm HHS diagnosis and monitor treatment rate; use the corrected sodium of 157.8 to select 0.45% NaCl for fluid therapy. 1, 2