Initial Confirmation of Hyponatremia Type
Hyponatremia workup should begin with confirmation of hypotonic hyponatremia and exclusion of pseudohyponatremia or translocational (nonhypotonic) causes. [1,2]
Immediate Clinical Assessment and Safety Checks
Evaluation should include assessment for neurologic symptoms and hemodynamic status because the workup influences urgency of treatment. [2,3]
Laboratory Workup to Classify Etiology
Serum studies
- Serum osmolality measurement is recommended to confirm hypotonicity versus isotonic or hypertonic hyponatremia. [1,2,4]
- Corrected sodium for hyperglycemia should be assessed when hyperglycemia is present because hypertonicity from glucose can cause apparent hyponatremia. [2,5]
- Serum chemistries should be reviewed to identify contributing solute or endocrine states, including glucose, blood urea nitrogen, creatinine, and thyroid/adrenal disorders when clinically indicated. [2,4,6]
Urine studies
- Urine osmolality should be measured because it distinguishes appropriately maximally dilute urine from impaired water excretion. [1,2,4]
- Urine sodium should be obtained because it helps differentiate effective arterial volume depletion from SIADH and other etiologies with impaired water excretion. [2,4,7]
Diagnostic Decision Framework Based on Osmolality and Urine Indices
Hypotonic hyponatremia with dilute urine
- If hypotonic hyponatremia is present with appropriately low urine osmolality, primary polydipsia or low-solute intake should be considered. [2,4]
Hypotonic hyponatremia with inappropriately concentrated urine
- If hypotonic hyponatremia is present with higher urine osmolality, ADH-mediated water retention etiologies should be prioritized. [2,4]
- Urine sodium low versus high should be used to support effective arterial volume depletion versus SIADH patterns. [2,4,7]
Assessment of Volume Status and Contributing Exposures
- Volume status should be assessed clinically because effective arterial volume depletion supports low urine sodium patterns in many cases. [2,7]
- Medication and iatrogenic fluid exposures should be reviewed because they commonly cause or worsen SIADH and related patterns. [2,6]
Endocrine and Renal Exclusions
- Adrenal insufficiency and hypothyroidism should be excluded when suggested by history, exam, or laboratory patterns because these conditions can produce hypotonic hyponatremia. [2,6,7]
Workup for Severe or Uncertain Cases
- Arterial or venous blood gas evaluation should be considered when acid–base status is abnormal because it can contribute to diagnostic classification. [4]
- Additional serum and urine solute testing should be considered when clinical patterns do not fit common categories, including assessment for unmeasured osmoles in suspected pseudohyponatremia or toxic ingestions. [1,2,7]
Practical Sampling Requirements
- Serum osmolality, urine osmolality, and urine sodium ideally should be obtained before initiating therapies that alter sodium or water balance, when feasible. [3]