|Nephrology|Intern

Hyponatremia Workup

A step-by-step approach to evaluating sodium disorders on the wards.

Hyponatremia is one of the most common electrolyte abnormalities you'll encounter on the wards. Here's the mental framework I use.

Step 1: Correct for Glucose

Before anything else, calculate the corrected sodium if glucose is elevated:

Corrected Na = Measured Na + 0.024 × (Glucose - 100)

If the corrected sodium is ≥135, you don't have true hyponatremia.

Step 2: Check Serum Osmolality

This tells you if you have true hypotonic hyponatremia:

  • < 280 mOsm/kg: Hypotonic (true hyponatremia) → continue workup
  • 280-295 mOsm/kg: Isotonic (pseudohyponatremia) → think lipids, proteins
  • > 295 mOsm/kg: Hypertonic → osmotically active substances (mannitol, contrast)

Step 3: Assess Volume Status

This is clinical—exam the patient:

| Finding | Volume Status | |---------|--------------| | Dry mucous membranes, poor skin turgor, tachycardia | Hypovolemic | | JVD, edema, ascites | Hypervolemic | | Normal exam | Euvolemic |

Step 4: Check Urine Studies

Order urine sodium and urine osmolality:

Urine Osmolality

  • < 100 mOsm/kg: ADH appropriately suppressed → primary polydipsia, beer potomania
  • > 100 mOsm/kg: ADH is active → proceed to urine sodium

Urine Sodium

  • < 30 mEq/L: Kidneys retaining sodium → volume depletion (GI losses, third-spacing) or edematous states (CHF, cirrhosis)
  • > 30 mEq/L: Kidneys wasting sodium → SIADH, diuretics, adrenal insufficiency, cerebral/renal salt wasting

The Pearl

Most hyponatremia on the medicine wards falls into three buckets:

  1. Hypovolemic with low urine Na → Give fluids
  2. Hypervolemic (CHF/cirrhosis) → Fluid restrict + treat underlying cause
  3. Euvolemic with high urine Na → Usually SIADH → Fluid restrict

Know these three patterns cold, and you'll handle 90% of cases.

What About Correction Rate?

For chronic hyponatremia (>48 hours or unknown duration), don't correct faster than 8-10 mEq/L in 24 hours to avoid osmotic demyelination syndrome.

For acute, symptomatic hyponatremia with seizures or severe symptoms, you can give hypertonic saline, but this requires ICU-level monitoring.

Try the Hyponatremia Calculator to walk through this framework with your patient's actual numbers.