What is the most likely diagnosis and initial treatment for a patient with serum sodium 128 mEq/L, urine sodium 22 mEq/L, urine osmolality 221 mOsm/kg, and serum osmolality 221 mOsm/kg?

Diagnosis: Hypovolemic Hyponatremia

This patient has hypovolemic hyponatremia, and the initial treatment is isotonic saline (0.9% NaCl) for volume repletion, with a target correction rate not exceeding 8 mmol/L in 24 hours. 1

Diagnostic Reasoning

The laboratory values point definitively toward hypovolemic hyponatremia:

• Serum sodium 128 mEq/L with serum osmolality 221 mOsm/kg confirms hypotonic hyponatremia 1
• Urine sodium 22 mEq/L (>20 mEq/L) suggests renal sodium losses rather than extrarenal losses, though values <30 mEq/L have a 71-100% positive predictive value for response to saline infusion 1
• Urine osmolality 221 mOsm/kg is inappropriately low relative to the hyponatremia, indicating some degree of impaired concentration but not the markedly elevated urine osmolality (>300-500 mOsm/kg) typical of SIADH 1, 2
• Isotonic urine-to-serum osmolality ratio (both 221) is unusual and suggests either dilute urine in the setting of volume depletion or a mixed picture 1

The combination of low serum sodium, relatively low urine sodium (<30 mEq/L threshold), and inappropriately dilute urine (not concentrated >500 mOsm/kg) strongly favors hypovolemic hyponatremia over SIADH 1, 3. In SIADH, you would expect urine osmolality >300 mOsm/kg and typically urine sodium >40 mEq/L 2, 3.

Initial Treatment Protocol

Volume Repletion Strategy

• Administer isotonic saline (0.9% NaCl) at an initial rate of 15-20 mL/kg/h, then adjust to 4-14 mL/kg/h based on clinical response 1
• Discontinue any diuretics immediately if the patient is on them 1
• Monitor for clinical signs of euvolemia: resolution of orthostatic hypotension, improved skin turgor, moist mucous membranes, stable vital signs 1

Sodium Correction Limits

• Maximum correction: 8 mmol/L in any 24-hour period to prevent osmotic demyelination syndrome 1
• Target correction rate: 4-8 mmol/L per day for standard-risk patients 1
• Check serum sodium every 4-6 hours during active correction 1

Monitoring Parameters

• Serum sodium, potassium, chloride, magnesium should be measured and corrected concurrently 1
• Urine output and urine sodium to assess response to volume repletion; successful repletion typically shows urine sodium <30 mmol/L 1
• Clinical volume status: orthostatic vital signs, skin turgor, mucous membrane moisture 1

Key Differentiating Features from SIADH

This is not SIADH because:

• Urine osmolality is not elevated (221 vs. expected >300-500 mOsm/kg in SIADH) 2, 3
• Urine sodium is borderline low (22 mEq/L vs. typically >40 mEq/L in SIADH) 2, 3
• The clinical picture suggests volume depletion rather than euvolemia 1

In SIADH, you would see inappropriately concentrated urine (>500 mOsm/kg), elevated urine sodium (>40 mEq/L), and euvolemic clinical status 2, 3. The treatment for SIADH would be fluid restriction to 1 L/day, which would be harmful in this hypovolemic patient 1, 2.

Critical Pitfalls to Avoid

• Do not use fluid restriction – this is appropriate for SIADH but will worsen hypovolemic hyponatremia 1
• Do not use hypotonic fluids (0.45% saline, lactated Ringer's, D5W) as they will worsen hyponatremia 1
• Do not exceed 8 mmol/L correction in 24 hours – overly rapid correction causes osmotic demyelination syndrome 1
• Do not use hypertonic saline (3%) unless severe neurological symptoms develop (seizures, coma, altered mental status) 1

Transition to Maintenance Phase

Once clinical euvolemia is achieved:

• Switch to maintenance isotonic fluids at 30 mL/kg/day for adults 1
• Reassess volume status to ensure no progression to hypervolemia 1
• If hyponatremia persists after volume repletion, consider alternative diagnoses such as SIADH or cerebral salt wasting 1