Nephrology Clinical Reference

Hyponatremia Classification Algorithm

ADH Stimulation
  1. Volume Depletion → ↓ EABV → ↑ ADH
  2. ADH-Like Effect (SIADH, drugs, pain)
Entry Criterion
Serum Na+ < 135 mEq/L
Step 1 · Measure Serum Osmolality
275 – 295 mOsm/kg
Isoosmolar
< 275 mOsm/kg
Hypoosmolar
> 295 mOsm/kg
Hyperosmolar
Pseudohyponatremia
  • Severe hypertriglyceridemia
  • Severe hyperproteinemia (MM, IVIG)
Lab artifact

Flame photometry falsely lowers Na. Direct ISE is accurate. No Na treatment needed.

True Hyponatremia
Continue to Step 2 ↓
Translocational Hyponatremia
  • Hyperglycemia (most common)
  • Mannitol infusion
  • Glycine / sorbitol (TURP syndrome)
Correction formula

Corrected Na = Measured Na + 1.6 × [(glucose − 100) / 100]

Hypoosmolar Hyponatremia — Step 2 · ADH Status via Uosm
2 True Hypotonicity < 270 mOsm/L
3
ADH Independent
Uosm < 100 mOsm/L
ADH-Independent Causes
  • Beer Potomania (low solute)
  • Primary Polydipsia (excess fluid)
  • Malnutrition / Tea-Toast (low solute)
Lab Findings

Una < 30 mmol/L

SG < 1.003

Uosm < 100 mOsm/L

3
ADH Dependent
Uosm > 100 mOsm/L
4 Assess volume status & urine sodium (Una)
Hypovolemic
Volume depletion → ADH ↑
  • GI Losses
  • Skin / Lung Losses
  • Renal Losses
  • Cerebral Salt Wasting
Una < 30 mmol/L

Non-Renal Loss

GI, Skin, Lung

Una > 30 mmol/L

Renal Loss

Diuretics, CSW

Euvolemic
ADH-like effect
  • SIADH
  • Nausea / Pain
  • Hypothyroidism
  • Hypocortisolism
  • Reset Osmostat
  • Medications (e.g. Ecstasy)
  • Cancer
Lab Findings

Una > 30 mmol/L

SG > 1.003

Uosm > 100 mOsm/L

Hypervolemic
ADH-like effect

Differentiate by Una:

Una > 30 mmol/L

AKI

CKD

Una < 30 mmol/L

CHF

Nephrotic

Cirrhosis

Lab Findings

Uosm > 100 mOsm/L

SG > 1.003