Specific Gravity Urine

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Specific gravity (which is directly proportional to urine osmolality which measures solute concentration) measures urine density, or the ability of the kidney to concentrate or dilute the urine over that of plasma. Dipsticks are available that also measure specific gravity in approximations. Most laboratories measure specific gravity with a refractometer.

Specific gravity between 1.002 and 1.035 on a random sample should be considered normal if kidney function is normal. Since the Specific Gravity of the glomerular filtrate in Bowman's space ranges from 1.007 to 1.010, any measurement below this range indicates hydration and any measurement above it indicates relative dehydration.

If sp gr is not > 1.022 after a 12 hour period without food or water, renal concentrating ability is impaired and the patient either has generalized renal impairment or nephrogenic diabetes insipidus. In end-stage renal disorder, Specific gravity tends to become 1.007 to 1.010.

Any urine having a specific gravity over 1.035 is either contaminated, contains very high levels of glucose, or the patient may have recently received high density radiopaque dyes intravenously for radiographic studies or low molecular weight dextran solutions. Subtract 0.004 for every 1% glucose to determine non-glucose solute concentration.

This test detects ion concentration of the urine. Small amounts of protein or ketoacidosis tend to elevate results of the specific gravity. Specific gravity is an expression of the weight of a substance relative to the weight of an equal volume of water. Water has a specific gravity of one. The specific gravity of your urine is measured by using a urinometer. Knowing the specific gravity of your urine is very important because the number indicates whether you are hydrated or dehydrated.

Additional conditions under which the test may be performed:
·Complicated UTI (pyelonephritis)  
·High blood sodium level  
·Low blood sodium level  
·Excessive urination  

Test results
Increases in specific gravity (high concentration of solutes in the urine) may be associated with dehydration, diarrhea, emesis, excessive sweating, glucosuria, renal artery stenosis, hepatorenal syndrome, decreased blood flow to the kidney (as a result of heart failure), and excess of antidiuretic hormone caused by Syndrome of inappropriate antidiuretic hormone. A specific gravity greater than 1.035 is consistent with frank dehydration. In neonates, normal urine specific gravity is 1.003. A specific gravity of > 1.009 is considered hypovolemia. Increased urine specific gravity may be due to:
·Addison's disorder (rare)  
·Diarrhea that causes dehydration  
·Heart failure (related to decreased blood flow to the kidneys)  
·Renal arterial stenosis  
·Syndrome of inappropriate antidiuretic hormone secretion (SIADH)  

Decreased specific gravity (low concentration of solutes in urine) may be associated with renal failure, pyelonephritis, diabetes insipidus, acute tubular necrosis, interstitial nephritis, and excessive fluid intake (e.g., psychogenic polydipsia). Decreased urine specific gravity may be due to:
·Aldosteronism (very rare)  
·Excessive fluid intake  
·Diabetes insipidus - central  
·Diabetes insipidus - nephrogenic  
·Renal failure  
·Renal tubular necrosis  
·Severe kidney infection (pyelonephritis)  

Also you should know
The role of the kidneys in humans and other mammals is to aid in the clearance of various water-soluble molecules, including toxins, toxicants, and metabolic waste. The body excretes some of these waste molecules via urination, and the role of the kidney is to concentrate the urine, such that waste molecules can be excreted with minimal loss of water and nutrients. The specific gravity of urine is the concentration of these excreted molecules in urine.

Osmolality is a more specific test for urine concentration. However, the specific gravity measurement is easier and more convenient and usually part of a routine urinalysis. It frequently makes the osmolality measurement unnecessary.

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