Serum Osmolality Calculation for SIADH Evaluation
Use the formula: 2 × [Na in mEq/L] + [glucose in mg/dL]/18 + [BUN in mg/dL]/2.8 to calculate serum osmolality when evaluating for SIADH. 1, 2
The Standard Formula
The most widely endorsed calculation across major guidelines is:
Serum Osmolality = 2 × Na (mEq/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8
This formula is recommended by:
- The American Diabetes Association for hyperglycemic crisis evaluation 1
- The American Academy of Pediatrics for fluid management 1
- Multiple clinical nutrition societies for hydration assessment 2, 3
Why This Formula Works for SIADH
For SIADH diagnosis specifically, you need to demonstrate hypo-osmolality (<275 mOsm/kg) alongside other criteria. 2, 4
The calculated osmolality helps you:
- Confirm plasma osmolality <275 mOsm/kg (required for SIADH) 2
- Rule out hyperglycemia or uremia as causes of altered mental status 1
- Establish that the hyponatremia is truly hypotonic 2
Alternative Formulas (When to Use Them)
If you need SI units (mmol/L for all values), use: Osmolarity = 1.86 × (Na + K) + 1.15 × glucose + urea + 14 2, 3
This European formula is recommended by Clinical Nutrition societies with a threshold of >295 mmol/L for hyperosmolality. 2 However, for SIADH evaluation in the U.S., stick with the standard formula above using conventional units.
The simplest bedside formula is: 2 × Na + glucose + BUN (all in mmol/L) 5
This Smithline-Gardner formula performs well across different analytical platforms and is useful for rapid mental calculation, though it slightly underestimates osmolality. 5
Critical SIADH Diagnostic Criteria Using Osmolality
Once you calculate serum osmolality, SIADH requires ALL of the following 2, 4:
- Serum sodium <134 mEq/L
- Plasma osmolality <275 mOsm/kg (your calculated value)
- Urine osmolality >500 mOsm/kg (or at minimum >100 mOsm/kg)
- Urinary sodium >20 mEq/L (often >40 mEq/L)
- Clinical euvolemia (no edema, no dehydration)
Important Caveats
You must measure actual serum osmolality in addition to calculating it when evaluating SIADH. 2 The difference between measured and calculated osmolality (the osmolal gap) helps identify unmeasured osmoles from toxins or other substances. 6, 5
Verify that glucose and BUN are within normal ranges before attributing abnormal osmolality solely to sodium disturbances. 2, 3 In hyperglycemia, you must correct the sodium value: add 1.6 mEq/L to measured sodium for every 100 mg/dL glucose above 100 mg/dL. 1, 2
Do not rely on clinical signs like skin turgor or mouth dryness to assess volume status in SIADH evaluation—these are highly unreliable. 2 Use objective laboratory values and careful clinical assessment of edema versus dehydration.
Normal Reference Range
Normal serum osmolality is 275-295 mOsm/kg. 1, 2, 3
Values <275 mOsm/kg indicate hypo-osmolality and warrant investigation for SIADH, overhydration, or other causes of hypotonic hyponatremia. 2 Values >300 mOsm/kg indicate hyperosmolality from dehydration or hyperglycemia. 2