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Urine osmolality in humans can range from approximately 50 to 1200 mOsm/kg, depending on whether the person has recently drunk a large quantity of water (the lower number) or has gone without water for a long time (the higher number). [2] Plasma osmolality with typical fluid intake often averages approximately 290 mOsm/kg H 2 O in humans. [2]
Urine osmolality in mOsm/kg, after fluid deprivation [3] After desmopressin [3] or vasopressin; Normal > 800 > 800 (<10% increase) a defect in ADH production (central/neurogenic diabetes insipidus) < 300 > 800 (>50% increase) a defect in the kidneys' response to ADH production (nephrogenic diabetes insipidus) < 300 < 500 (<50% increase)
A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma. Syndrome of inappropriate ADH secretion occurs when excessive release of antidiuretic hormone results in inappropriately elevated urine osmolality (>100 mOsmol/L) relative to the blood plasma, leading to hyponatraemia.
Reference ranges for urine tests are described below: Measurement Lower limit Upper limit ... per High Power Field (HPF) RBC casts: n/a 0 / negative [2] White blood ...
The calculated osmolality is then subtracted from the measured osmolality to provide the osmol gap, or the difference between these two values. If this gap falls within an acceptable range,(<10) then it is assumed that sodium, glucose, BUN are indeed the major dissolved ions and molecules in the serum.
A urinalysis will typically show a decreased urine sodium level, a high urine creatinine-to-serum creatinine ratio, a high urine urea-to-serum urea ratio, and concentrated urine (determined by osmolality and specific gravity). None of these is particularly useful in diagnosis.
Diagnosis is based on clinical and laboratory findings of low serum osmolality and low serum sodium. [13] Urinalysis reveals a highly concentrated urine with a high fractional excretion of sodium (high sodium urine content compared to the serum sodium). [14] A suspected diagnosis is based on a serum sodium under 138.
In primary polydipsia, the urine osmolality should increase and stabilize at above 280 mOsm/kg with fluid restriction, while a stabilization at a lower level indicates diabetes insipidus. [18] Stabilization in this test means, more specifically, when the increase in urine osmolality is less than 30 Osm/kg per hour for at least three hours. [18]