Urine Sodium in Hyponatremia: Diagnostic Interpretation
Urine sodium is the single most critical laboratory value for determining whether hyponatremia results from renal versus extrarenal causes and for distinguishing between the major categories of hyponatremia—particularly SIADH versus hypovolemic states. 1
Primary Diagnostic Thresholds
Urine sodium <30 mmol/L strongly predicts hypovolemic hyponatremia with a positive predictive value of 71-100% for response to isotonic saline infusion. 1, 2 This indicates the kidneys are appropriately conserving sodium in response to volume depletion from extrarenal losses (vomiting, diarrhea, burns, dehydration). 1, 3
Urine sodium >20-40 mEq/L with elevated urine osmolality (>300-500 mosm/kg) in a euvolemic patient indicates SIADH. 1, 2 This reflects the pathophysiology of SIADH: inappropriate ADH activity causes water retention, which triggers physiologic natriuresis to maintain fluid balance despite euvolemia. 3
Urine sodium >20 mEq/L with signs of volume depletion suggests renal salt wasting—either from diuretics, adrenal insufficiency, salt-losing nephropathy, or cerebral salt wasting in neurosurgical patients. 1, 2
Refined Diagnostic Cutpoint
Recent evidence challenges traditional teaching: urine sodium values up to 50 mEq/L can still demonstrate clinically meaningful responses to isotonic saline in hypovolemic hyponatremia. 4 A cutpoint of 50 mEq/L provides optimal diagnostic accuracy (sensitivity 0.89, specificity 0.69) for separating SIADH from hypovolemia when the clinical picture is unclear. 4
Critical Context: Volume Status Assessment
Physical examination alone has poor accuracy for determining volume status (sensitivity 41.1%, specificity 80%), making urine sodium interpretation essential but requiring integration with other findings. 1, 2 Look specifically for:
- Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
- Hypervolemic signs: peripheral edema, ascites, jugular venous distention, pulmonary congestion 1, 3
- Euvolemic state: absence of both hypovolemic and hypervolemic findings 1
Diagnostic Algorithm for Hyponatremia
Step 1: Confirm true hypotonic hyponatremia by checking serum osmolality (normal 275-290 mosm/kg) to exclude pseudohyponatremia. 1
Step 2: Assess volume status clinically (recognizing its limitations). 1, 2
Step 3: Measure urine sodium and urine osmolality simultaneously:
- Urine Na <30 mmol/L + hypovolemic signs → Extrarenal losses; treat with isotonic saline 1, 2
- Urine Na >40 mEq/L + euvolemic + urine osm >300 mosm/kg → SIADH; treat with fluid restriction 1, 2
- Urine Na >20 mmol/L + hypovolemic signs → Renal salt wasting (diuretics, adrenal insufficiency, cerebral salt wasting) 1, 2
- Urine Na >20 mmol/L + hypervolemic signs → Heart failure, cirrhosis, or advanced renal failure 1, 3
Step 4: In neurosurgical patients, distinguish SIADH from cerebral salt wasting using central venous pressure if available (CVP <6 cm H₂O indicates CSW). 1, 2
Complementary Laboratory Values
Serum uric acid <4 mg/dL has 73-100% positive predictive value for SIADH (though may also occur in cerebral salt wasting). 1, 2 This reflects increased renal uric acid excretion from volume expansion in SIADH. 1
Urine osmolality interpretation:
- <100 mosm/kg indicates appropriate ADH suppression (primary polydipsia) 1
300-500 mosm/kg indicates impaired free water excretion (SIADH, heart failure, cirrhosis) 1, 3
Common Pitfalls
Never rely on urine sodium alone without assessing volume status—both SIADH and cerebral salt wasting present with elevated urine sodium, but require opposite treatments (fluid restriction vs. volume replacement). 1, 2
Avoid misinterpreting urine sodium 30-50 mEq/L as automatically indicating SIADH—many hypovolemic patients in this range still respond to saline. 4
Do not obtain ADH or natriuretic peptide levels—these are not supported by evidence and delay treatment. 1
In hypervolemic states (heart failure, cirrhosis), urine sodium reflects diuretic use and sodium intake rather than volume status, making interpretation more complex. 1, 5