Specific gravity, in the context of clinical pathology, is a

urinalysis Urinalysis, a portmanteau of the words ''urine'' and ''analysis'', is a panel of medical tests that includes physical (macroscopic) examination of the urine, chemical evaluation using urine test strips, and microscopic examination. Macroscopic e ...

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 The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...

s in humans and other mammals is to aid in the clearance of various water-soluble molecules, including

toxins A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849–1 ...

, toxicants, and

metabolic waste Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, ...

. The body excretes some of these waste molecules via urination, and the role of the kidney is to

concentrate A concentrate is a form of Chemical substance, substance that has had the majority of its base component (in the case of a liquid: the solvent) removed. Typically, this will be the removal of water from a Solution (chemistry), solution or suspens ...

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 Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...

. 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 In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...

diarrhea Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin wi ...

, emesis, excessive

sweating Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammals. Two types of sweat glands can be found in humans: eccrine glands and apocrine glands. The eccrine sweat glands are distr ...

, urinary tract/bladder infection,

glucosuria Glycosuria is the excretion of glucose into the urine. Ordinarily, urine contains no glucose because the kidneys are able to reabsorb all of the filtered glucose from the tubular fluid back into the bloodstream. Glycosuria is nearly always caused ...

, renal artery stenosis,

hepatorenal syndrome Hepatorenal syndrome (often abbreviated HRS) is a life-threatening medical condition that consists of rapid deterioration in kidney function in individuals with cirrhosis or fulminant liver failure. HRS is usually fatal unless a liver transplan ...

, 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, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...

), and an excess of

antidiuretic hormone Human vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the AVP gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It then travel ...

caused by the syndrome of inappropriate antidiuretic hormone secretion. A specific gravity greater than 1.035 is consistent with frank dehydration. In neonates, normal urine specific gravity is 1.003.

Hypovolemic Hypovolemia, also known as volume depletion or volume contraction, is a state of abnormally low extracellular fluid in the body. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the los ...

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 end-stage kidney disease, 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 failure is classified as eit ...

pyelonephritis Pyelonephritis is inflammation of the kidney, typically due to a bacterial infection. Symptoms most often include fever and flank tenderness. Other symptoms may include nausea, burning with urination, and frequent urination. Complications may ...

diabetes insipidus Diabetes insipidus (DI), recently renamed to Arginine Vasopressin Deficiency (AVP-D) and Arginine Vasopressin Resistance (AVP-R), is a condition characterized by large amounts of dilute urine and increased thirst. The amount of urine produce ...

acute tubular necrosis Acute tubular necrosis (ATN) is a medical condition involving the death of tubular epithelial cells that form the renal tubules of the kidneys. Because necrosis is often not present, the term acute tubular injury (ATI) is preferred by pathologists ...

interstitial nephritis Interstitial nephritis, also known as tubulointerstitial nephritis, is inflammation of the area of the kidney known as the renal interstitium, which consists of a collection of cells, extracellular matrix, and fluid surrounding the renal tubules. ...

, and excessive fluid intake (e.g.,

psychogenic polydipsia Primary polydipsia and psychogenic polydipsia are forms of polydipsia characterised by excessive fluid intake in the absence of physiological stimuli to drink. Psychogenic polydipsia which is caused by psychiatric disorders, often schizophrenia, i ...

).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