Urine Osmolality

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The osmolality urine test measures the concentration of scruples in urine. This test helps examine your body's water balance and urine concentration. Osmolality is a more exact measurement of urine concentration than the urine specific gravity test.
Osmolality (particles/kg water) and osmolarity (particles/liter of solution) are sometimes confused, but for rarefied fluids such as urine they are basically the same.
The higher the number of urine millosmoles the more concentrated the urine; this is the expected physiological response to dehydration.
A blood serum test may also be done to measure osmolality.

Test results
High value measurements may indicate:
·Addison's disorder (rare)  
·Congestive heart failure  
·Renal artery stenosis  
·Syndrome of inappropriate ADH secretion  
Low value measurements may indicate:
·Aldosteronism (very rare)  
·Diabetes insipidus (rare)  
·Excess fluid intake  
·Kidney failure  
·Renal tubular necrosis  
·Severe pyelonephritis  
Additional conditions under which the test can be done:
·Complicated urinary tract infection (pyelonephritis)  
·High blood sodium level  
·Low blood sodium level  
·Excessive urination  

Also you should know
Osmolality in urine is usually used to research: renal function, activity, polyuria and oliguria. Normal kidneys may concentrate urine to an osmolality four times greater than blood serum. They can also dilute urine to quarter the osmolality of serum. Individuals with impaired renal function may do not have ability to concentrate urine. As a result, urine osmolality can fall to approach that of serum, approximately 290mOsm/Kg.

In the healthy body, osmoreceptors in the hypothalmus sense the diffusion of water out or into of receptor cells caused by changes in blood serum osmolality. As a response, the hypothalamus directs the mucilaginous to increase or decrease the release of vasopressin from the posterior mucilaginous. The release of vasopressin causes increased water desorption in the distal tubules and collecting ducts of the kidney. This decreases water loss and concentrates urine. However, dehydration increases vasopressin release leading to water reservation and urine concentration. Fluid overload reduces vasopressin release which leads to diuresis.

All information on this page is intended for your general knowledge only and does not provide medical advice, diagnosis or treatment.