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One of the important functions of the kidneys is to remove metabolic waste from the body while maintaining the composition and volume of fluids in the body. The kidneys filter water and other particles from the blood in the formation of urine. The process is a dynamic exchange of these products, which is influenced by hormones resulting in dilution or concentration of the urine as needed by the body.
A urinalysis usually tests for the presence of blood cells, proteins, glucose and other particles or by-products in the urine. However, urine specific gravity indicates the density or the concentration of these particles in the urine, which may indicate the presence of a disease that alters the ability of the kidneys to concentrate the urine.
The concentration of the particles in the urine is measured by the test for urine specific gravity. It may also be called urine density and more specifically, urine osmolality, which is expressed in particles/kg water.
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What Urine Specific Gravity Results Mean
The normal specific gravity of urine is 1.020 to 1.028, although values may differ slightly in different laboratories or institutions.
A urinalysis which shows a markedly decreased specific gravity (below 1.002) may indicate an increased body fluid content which is not eliminated efficiently, thus producing dilute urine. Conditions that may bring about a low urine specific gravity are:
- Excessive fluid intake
- Diabetes insipidus – not the same as diabetes mellitus, this disorder affects either the brain or the kidneys, resulting in excessive thirst, water intake and urine formation.
- Renal failure – kidneys fail to reabsorb water due to kidney disease
- Renal tubular necrosis – damaged kidneys resulting from injury, trauma, excessive blood loss and shock and blood transfusion reaction
- Severe pyelonephritis or kidney infection
- Aldosteronism – a rare hormonal disorder that results in increased blood pressure, excessive loss of potassium and sodium retention
When lab tests show an increased specific gravity (greater than 1.028), highly concentrated urine may be the result of any of the following conditions:
- Dehydration – may be the result of blood loss, diarrhea, excessive sweating, excessive vomiting or water restriction
- Heart failure – results in decreased kidney perfusion and decreased urine output
- Glucose in the urine, as in diabetics
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH) - usually due to a traumatic brain injury or brain tumors
- Renal arterial stenosis – narrowing of the artery supplying blood to the kidneys, resulting in decreased perfusion and urine concentration
- Addison's disease – a rare condition where the adrenal glands produce too little cortisol and aldosterone, hormones that affect urine formation
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Lab Testing for Urine Concentration
When the results of urine specific gravity are unusually high or low, a urine concentration test may be done to measure the ability of the kidneys to conserve or excrete water appropriately. In this test the specific gravity of urine is tested before and after a challenge is performed.
The ability of the kidneys to concentrate urine or to retain body fluids is measured after a water loading challenge, where the patient is made to drink excessive amounts of water or receive fluids through a vein. Conversely, the kidneys may be tested for the ability to produce urine after water deprivation. Injecting antidiuretic hormone (ADH), which should cause the urine to become concentrated, is another challenge that can be done to test one’s kidney function.
Comparing the results of the test for specific gravity before and after a challenge will reveal if the kidneys are functioning optimally under controlled circumstances and can lead to the diagnosis of disease or indicate the need for further lab work-up to determine the cause of disease.
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- Med Help, “Urine Specific Gravity" accessed 2/2/11
- Medline Plus, “Urine Concentration Test" accessed 2/2/11
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