Laboratory Workup for Hyponatremia
Begin with serum osmolality to confirm true hypotonic hyponatremia, then measure urine osmolality and urine sodium concentration alongside clinical volume-status assessment to determine the underlying cause. 1, 2
Initial Laboratory Tests
Step 1: Confirm True Hyponatremia
- Serum osmolality – measure to exclude pseudohyponatremia (normal osmolality) and translocational hyponatremia (elevated osmolality from glucose, mannitol, or glycine). 1, 3, 4
- True hypotonic hyponatremia is confirmed when serum osmolality is < 280 mOsm/kg. 4
Step 2: Assess Renal Water Handling
- Urine osmolality – distinguishes appropriate ADH suppression (< 100 mOsm/kg) from inappropriate ADH activity (> 100 mOsm/kg). 1, 4
- A urine osmolality < 100 mOsm/kg indicates complete suppression of antidiuretic hormone and suggests primary polydipsia or reset osmostat. 4
- A urine osmolality > 100 mOsm/kg indicates impaired water excretion and requires further evaluation with urine sodium. 1, 4
Step 3: Determine Volume Status and Sodium Handling
Urine sodium concentration – the critical test to differentiate causes once impaired water excretion is confirmed. 1, 2, 4
Serum uric acid – a level < 4 mg/dL provides 73–100% positive predictive value for SIADH versus hypovolemia. 5
Supplementary Laboratory Tests
Hormonal Evaluation
- Thyroid-stimulating hormone (TSH) – to exclude hypothyroidism-related euvolemic hyponatremia. 5, 4
- Morning cortisol and ACTH – to rule out adrenal insufficiency, which can present as euvolemic or hypovolemic hyponatremia. 5, 4
Electrolyte and Metabolic Panel
- Serum potassium – correct hypokalemia as it worsens hyponatremia and must be addressed simultaneously. 6
- Serum creatinine and BUN – assess renal function and calculate BUN/creatinine ratio (elevated in hypovolemia). 7, 5
- Serum glucose – to calculate corrected sodium and exclude hyperglycemic hyponatremia. 3
Additional Tests in Specific Contexts
- Arterial blood gases – in difficult cases to assess acid-base status. 4
- Lipid panel and protein levels – if pseudohyponatremia from severe hyperlipidemia or hyperproteinemia is suspected. 3
- Infection screening (cultures, inflammatory markers) – acute inflammation can precipitate hyponatremia. 5
Point-of-Care Adjuncts to Laboratory Testing
Bedside Ultrasound
- Inferior vena cava (IVC) diameter and collapsibility – helps estimate intravascular volume status when physical examination is equivocal. 5, 8
- Lung ultrasound for B-lines – detects pulmonary interstitial fluid indicating volume overload. 5
Physical Examination Findings to Document
- Orthostatic vital signs – > 10% pulse increase or > 10% systolic blood pressure drop suggests hypovolemia. 5
- Jugular venous distention, peripheral edema, ascites, pulmonary edema – indicate hypervolemia. 7, 5
- Physical examination alone has only 41–48% accuracy for volume status; combine with laboratory and ultrasound data. 5
Monitoring Parameters During Workup
- Serial serum sodium – check every 4–6 hours in symptomatic patients, daily in asymptomatic cases. 6
- Daily weight and strict intake/output – track volume changes during evaluation and treatment. 7
- Repeat urine sodium and osmolality – if initial results are inconclusive or clinical picture changes. 1
Common Diagnostic Pitfalls to Avoid
- Do not rely on physical examination alone – it correctly classifies volume status in only about half of cases; always combine with urine sodium, serum uric acid, and point-of-care ultrasound. 5
- Do not miss pseudohyponatremia – always measure serum osmolality first to exclude artifactual hyponatremia from hyperlipidemia or hyperproteinemia. 3
- Do not overlook medication history – diuretics (especially thiazides), SSRIs, carbamazepine, and NSAIDs are common culprits. 3
- Do not forget to measure both urine osmolality and urine sodium – urine sodium alone is insufficient; you need both to navigate the diagnostic algorithm. 1, 4
- Do not assume SIADH without excluding hypothyroidism and adrenal insufficiency – these are treatable causes that mimic SIADH. 4