Tests
1st tests to order
serum electrolyte panel with glucose, BUN, and creatinine
Test
Should be ordered in all patients with suspected hypernatremia.
A serum sodium concentration >145 mEq/L confirms presence of hypernatremia.[1] Severe hypernatremia has variously been defined as a serum sodium concentration of >152 mEq/L, >155 mEq/L, or >160 mEq/L.[2][3][4] However, there is no consensus as to the exact level. Extremely high sodium levels occur in salt poisoning.
May reveal other electrolyte abnormalities, renal impairment, or uncontrolled diabetes mellitus. Some patients may have hypokalemia or, more rarely, hypercalcemia.
Patients with hypernatremia often have a high BUN and/or creatinine level. The high BUN level may worsen the hypernatremia by causing an osmotic diuresis.[51][68]
Result
serum sodium >145 mEq/L; other parameters variable
urine osmolality
Test
Should be ordered in all patients with hypernatremia, as it may help determine the underlying etiology.
Urine osmolality ≤plasma osmolality (low; <150 mOsm/kg): suggests diabetes insipidus.
Urine osmolality ≥plasma osmolality (high; >500 mOsm/kg): suggests pure volume depletion not due to diabetes insipidus (e.g., gastrointestinal or insensible losses).
Urine osmolality not too different from plasma osmolality: suggests a renal concentrating defect most commonly due to renal failure, osmotic diuresis, and/or use of diuretics.
Can be variable in patients with iatrogenic hypernatremia.
Result
<150 mOsm/kg: diabetes insipidus; 200-500 mOsm/kg: renal concentrating defect; >500 mOsm/kg: pure volume depletion
serum osmolality
Test
Unlike hyponatremia, hypernatremia is always associated with serum hyperosmolality.
Result
>295 mOsm/kg
urine electrolytes
Test
Should be ordered in patients with urinary losses to determine electrolyte-free water excretion.
Result
variable
urine flow rate
Test
Volume of urine excreted in a specific time period (e.g., over 30-60 minutes).
Should be ordered in patients with urinary losses to determine electrolyte-free water excretion.
Result
variable
electrolyte-free water excretion
Test
Should be calculated in patients with urinary losses.[Figure caption and citation for the preceding image starts]: Electrolyte-free water excretion formula. V = urine flow rate. UNa = urine concentration of sodium (mEq/L). UK = urine concentration of potassium (mEq/L). PNa = plasma concentration of sodium (mEq/L)Created by the BMJ Knowledge Centre [Citation ends].
The electrolyte-free water excretion (also known as electrolyte-free water clearance) value indicates how much electrolyte-free water is being lost through the urine at any given time.[64][68][77] However, it does not provide a value for the total amount of free water needed to correct the hypernatremia.[78]
Result
low (<0.5 L/day): suggests inadequate free water intake; high (≥1 L/day): suggests large free water losses; very high (>5 L/day): suggests diabetes insipidus
Tests to consider
desmopressin challenge test
Test
Should be ordered in patients with suspected diabetes insipidus.
Differentiates between central and nephrogenic diabetes insipidus.
Patients are given a standard dose of desmopressin, and serum osmolality, urine osmolality, and urine volumes are measured hourly over a 4-hour period.
Patients with central diabetes insipidus respond to desmopressin with a reduction in urine output and increased urine osmolality. Patients with nephrogenic diabetes insipidus have little or no response.
Result
decreased urine output; increased urine osmolality (central diabetes insipidus)
serum arginine vasopressin (AVP) level
Test
May be useful to help distinguish central diabetes insipidus from nephrogenic diabetes insipidus.
Result
low (central diabetes insipidus)
MRI or CT brain
Test
Recommended in all patients with central diabetes insipidus to determine the underlying cause.
Result
may show pituitary tumor or other abnormalities
Serum creatine phosphokinase (CPK)
Test
May be helpful in evaluating the underlying cause, particularly hypernatremia associated with rhabdomyolysis.
Result
elevated in hypernatremia associated with rhabdomyolysis
Renal ultrasound
Test
May be helpful in evaluating the underlying cause, particularly to rule out obstructive uropathy in patients with renal disease.
Result
may show hydronephrosis in hypernatremia associated with obstructive uropathy
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