Urine Osmolality
Urine osmolality is used to measure the number of dissolved particles per unit of water in the urine. As a measure of urine concentration, it is more accurate than specific gravity. Urine osmolality is useful in diagnosing disorders of urinary concentration such diabetes insipidous and in assessing hydration status. Often, the assessment of any disorder involving antidiuretic hormone (ADH) will require both serum and urine osmolality to assess concentrating ability of the kidney.
Normal urine osmolality is as follows [1] :
12- to 14-hour fluid restriction: >850 mOsm/kg H 2O (SI units)
Random specimen: 50-1200 mOsm/kg H 2O, depending on fluid intake, or 50-1200 mmol/kg (SI units)
Interpretation
An individual with a normal diet and normal fluid intake has a urine osmolality of approximately 500-850 mOsm/kg water. After age 20 years, the upper level of the reference range declines by about 5 mOsm/kg/year.
In the setting of excess fluid intake, a healthy kidney can concentrate urine to 800-1,400 mOsm/kg of water; the minimal osmolality is 40-80 mOsm/kg of water. In the setting of dehydration, the urine osmolality should exceed the plasma osmolality 3- to 4-fold.
The following are associated with increased urine osmolality:
Collection: Random urine urine sample
Specimen preparation: 1-mL aliquot transferred to laboratory; need minimum of 0.5 mL
Storage/transport: Frozen; on ice
Results: Should be expected in 24 hours [2]
Background
Definition
Urine osmolality is used to measure the number of dissolved particles per unit of water in the urine. As a measure of urine concentration, it is more accurate than specific gravity. Urine osmolality is useful in diagnosing renal disorders of urinary concentration and dilution and in assessing status of hydration.
Clinical relevance
Osmolality indicates solute concentration. Urine osmolality is an index of the concentration of osmotically active particles, particularly chloride, sodium, urea, and potassium; glucose can also add significantly to the osmolality when it is abundant in urine.
In a healthy state, the specific gravity of the urine corresponds to the urine osmolality. An evaluation of the osmolality, either routinely or under artificial conditions, indicates the kidney’s ability to maintain tonicity and water balance of the extracellular fluid.
A comparison of the urine osmolality to the serum osmolality yields additional information about water handling by the kidney or abnormalities of urine concentration or dilution, as does electrolyte studies.
Typically, the ratio of urine osmolality to serum osmolality is 1-3.
Method description
Measuring the freezing point of urine is the most commonly used method in osmometry. The extent to which urine can be cooled to less than 0°C reflects the concentration of substances dissolved in the urine. One mOsm/kg of water decreases the freezing point by 0.001858°C.
Description and clinical significance
Variations in urine osmolality play a principal role in regulating the Na+ concentration and plasma osmolality. Osmoreceptors in the hypothalamus mediate this response, influencing both secretion of ADH and thirst.
Urine osmolality in increased water load of body
After a water load, the plasma osmolality drops transiently, thereby suppressing the release of ADH. This decreases reabsorption of water in the collecting tubules, causing excretion of the excess water in dilute urine.
Urine osmolality in water restriction
Conversely, water restriction increases the plasma osmolality, ADH secretion, and renal water reabsorption, in that order, causing water retention and excretion of concentrated urine.
Urine osmolality and hyponatremia
Hypoosmolality with hyponatremia should virtually abolish ADH release. This should result in excretion of maximally dilute urine, with the urine osmolality falling below 100 mOsm/kg of water. In this setting, the hyponatremia is probably caused by intake of excess water that exceeds normal excretory capacity, a rare condition called primary polydipsia.
However, it is much more common for the urine osmolality to be inappropriately high, and the kidneys’ inability to excrete water normally results in hyponatremia. This is most commonly caused by unrestrained ADH release due to SIADH or volume depletion.
Urine osmolality and hypernatremia
Hypernatremia should stimulate ADH secretion, with the urine osmolality exceeding 600-800 mOsm/kg of water. If the urine is concentrated, the elevated plasma Na+ concentration is due to extrarenal water loss (from the skin or respiratory tract) or administration of Na+ in excess of water the plasma Na+ concentration.
Conversely, a plasma osmolality higher than that of urine indicates primary renal water loss caused by ADH lack or resistance.
Urine osmolality and acute tubular necrosis
The urine osmolality (as well as the fractional excretion of sodium [FENa]) may also help differentiate volume depletion from postischemic acute tubular necrosis (ATN) as the etiology of acute renal failure. Because hypovolemia greatly stimulates ADH release, ADH levels tend to be high. In contrast, tubular dysfunction in acute tubular necrosis impairs the response to ADH, resulting in excretion of urine with an osmolality typically less than 400 mOsm/kg of water. [3, 4]
In comparison, the urine osmolality in hypovolemia without underlying renal disease may be more than 500 mOsm/kg of water. Thus, a high urine osmolality essentially rules out acute tubular necrosis.
However, if the urine is found to be isosmotic, it is less diagnostically useful. This is consistent with acute tubular necrosis but does not exclude volume depletion, as the concentrating ability may also be impaired. This is common in elderly patients and in patients with a severely reduced glomerular filtration rate.