Overview
The urine specific gravity is a measure of the amount of solute compared to water. Pure water has a specific gravity of 1.000.
This is an indicator of the kidney's ability to concentrate or dilute urine, and correlates with the urine osmolality.
Normal Range
- 1.002 - 1.035
Increased Urine Specific Gravity
If the urine specific gravity is reduced then this reflects an inability to produce dilute urine. As a result, the urine is excessively concentrated.
Causes of Increased Urine Specific Gravity
Increased Electrolytes
- Dehydration - poor intake, GI losses, diuresis
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Adrenal insufficiency
- Hypervolaemic hyponatraemia - liver cirrhosis, congestive cardiac failure, nephrotic syndrome
Other Solutes
- Glycosurea (diabetes)
- Proteinuria (e.g. glomerulonephropathy, myeloma)
Reduced Urine Specific Gravity
If the urine specific gravity is reduced, this reflects an inability to concentrate urine. As a result, the urine is excessively diluted.
Causes of Reduced Urine Specific Gravity
- Polydipsia (excessive fluid consumption)
- Diabetes insipidus
- Pyelonephritis
- Acute tubular necrosis (polyuric phase)
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