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
One of the main roles of the
kidney
In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
s 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
Urination is the release of urine from the bladder through the urethra in Placentalia, placental mammals, or through the cloaca in other vertebrates. It is the urinary system's form of excretion. It is also known medically as micturition, v ...
, 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 concentration of the excreted molecules determines the urine's
specific gravity. In adult humans, normal specific gravity values range from 1.010 to 1.030.
Specific gravity and disease
Adults generally have a specific gravity in the range of 1.010 to 1.030. Increases in specific gravity (hypersthenuria, i.e. increased concentration of solutes in the urine) may be associated with
dehydration,
diarrhea,
emesis, excessive
sweating,
urinary tract/bladder infection,
glucosuria,
renal artery stenosis,
hepatorenal syndrome, decreased blood flow to the kidney (especially as a result of
heart failure
Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome caused by an impairment in the heart's ability to Cardiac cycle, fill with and pump blood.
Although symptoms vary based on which side of the heart is affected, HF ...
), and an excess of
antidiuretic hormone caused by the
syndrome of inappropriate antidiuretic hormone secretion
Syndrome of inappropriate antidiuretic hormone secretion (SIADH), also known as the syndrome of inappropriate antidiuresis (SIAD), is characterized by a physiologically inappropriate release of Vasopressin, antidiuretic hormone (ADH) either from ...
. A specific gravity greater than 1.035 is consistent with frank dehydration. In neonates, normal urine specific gravity is 1.003.
Hypovolemic patients usually have a specific gravity >1.015.
Decreased specific gravity (hyposthenuria, i.e. decreased concentration of solutes in urine) may be associated with
renal failure
Kidney failure, also known as renal failure or end-stage renal disease (ESRD), is a medical condition in which the kidneys can no longer adequately filter waste products from the blood, functioning at less than 15% of normal levels. Kidney fa ...
,
pyelonephritis,
diabetes insipidus,
acute tubular necrosis,
interstitial nephritis, and excessive fluid intake (e.g.,
psychogenic polydipsia).
Explanation of Interstitial Nephritis on MedlinePlus
/ref>
Osmolality is normally used for more detailed analysis, but USG remains popular for its convenience.
References
{{Urine tests
Urine tests
Laboratory techniques