Specific gravity (kidney)

Overview
Specific gravity, in the context of clinical pathology, is a urinalysis parameter commonly used in the evaluation of kidney function and can aid in the diagnosis of various renal diseases.

Background
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, generally expressed in grams/milliliter (g/ml). In humans, normal specific gravity values range from 1.002 to 1.028 g/ml

Specific gravity and disease
Increases in specific gravity (i.e. increased 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.

Decreased specific gravity (i.e. decreased 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).