For explanation of the ratio of densities of two materials, see Relative density.
Reading of a urine specific gravity of ~1.024 via a handheld refractometer. SG measurement is taken by reading the boundary between the dark and light fields against the graduations on the left column. |
| evaluation of kidney function |
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[edit]
One of the main roles 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 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[edit]
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), and an excess of antidiuretic hormone caused by the syndrome of inappropriate antidiuretic hormone secretion.[1] A specific gravity greater than 1.035 is consistent with frank dehydration.[2] 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, pyelonephritis, diabetes insipidus, acute tubular necrosis, interstitial nephritis, and excessive fluid intake (e.g., psychogenic polydipsia).[3][4]
Osmolality is normally used for more detailed analysis, but USG remains popular for its convenience.[5]
References[edit]
- ^ Explanation of Hepatorenal Syndrome on MedlinePlus
- ^ Armstrong, Lawrence E.; Herrera Soto, Jorge A.; Hacker, Frank T. Jr.; Casa, Douglas J.; Kavouras, Stavros A.; Maresh, Carl M. (December 1998). "Urinary indices during dehydration, exercise, and rehydration" (PDF). International Journal of Sport Nutrition. 8 (4): 345–355. doi:10.1123/ijsn.8.4.345. PMID 9841955.
- ^ Explanation of Acute Tubular Necrosis on MedlinePlus
- ^ Explanation of Interstitial Nephritis on MedlinePlus
- ^ "Urine specific gravity". MedlinePlus : U.S. National Library of Medicine. Retrieved 31 May 2013.
A urine specific gravity test compares the density of urine with the density of water. It allows doctors to see the concentration of particles present in urine.
The test involves collecting a urine sample for testing in a laboratory.
A doctor will use a urine specific gravity test if they suspect that a person is dehydrated, has impaired kidney function, or has a condition that can alter the body’s fluid status, such as diabetes insipidus.
Doctors consider the result of a urine specific gravity test to be normal if it is within the range 1.003–1.030.
However, doctors cannot specify low, acceptable, or high values because many factors can affect urine composition. Urine specific gravity is only one measure of health.
If a doctor identifies an unusual result, they may recommend further testing to determine if an underlying condition is causing the urine changes.
Additional urine testing may include:
- urine culture
- urine osmolality
- urine pH
- ketone testing
Results from these other tests will help make an accurate diagnosis.
Learn more about urinalysis here.
Causes of high readings
High specific gravity suggests that the concentration of urine is too high. This can be a sign of dehydration, and the doctor may recommend drinking more clear fluids.
Conditions that cause high specific gravity include:
- dehydration
- diarrhea or vomiting resulting in dehydration
- congestive heart failure
- shock
High levels of certain substances in the urine can also cause high specific gravity.
Substances include:
- glucose
- proteins
- red or white blood cells
Learn more about red blood cells in the urine here.
Causes of low readings
Low specific gravity suggests that urine is too diluted. The person may be drinking too much fluid or have a condition that makes them thirsty.
Conditions that cause low specific gravity include:
- diabetes insipidus
- kidney damage or kidney failure
- drinking too much fluid, for example, due to excess thirst, or polydipsia
- using diuretics, or water pills, which cause the body to remove extra sodium from the urine
- congestive heart failure
Hyponatremia can result from some of these conditions. If a person has hyponatremia, sodium levels in the body are too low.
Learn what happens where there is too much sodium in the body.
Other factors that can affect results
Other factors that may give an inaccurate result include :
- exposing the sample to light and temperature, which can change the composition of some particles
- bacterial growth
- alkaline pH
- glucose, which can affect pH
- contrast agents used in scans and other procedures
- recent exercise
- some foods, such as beetroot, rhubarb, blackberries, and food colorings
- some medications, including ibuprofen, chloroquine, and metronidazole
Learn more about the pH levels of urine here.
Urine specific gravity and urine color
If a person has concerns about whether they are consuming enough fluid, checking their urine color may help. Urine tends to be darker and has higher specific gravity when a person does not drink enough fluids.
Urine is lighter and has lower specific gravity when a person is well-hydrated.
If a person continues to have dark urine after consuming extra fluid, it may be time to speak with a doctor.
Urine specific gravity tests can give a more precise measurement of the urine’s concentration than just looking at the color alone.
Learn more about what different colors of urine can mean.
If urine specific gravity is high due to dehydration, a doctor may recommend:
- drinking more clear fluids, especially water
- consuming foods containing water, such as soups or jello
- having intravenous fluids if the person is unable to take fluids by mouth
If urine specific gravity is high for another reason, such as diarrhea or shock, the doctor will investigate and treat the underlying cause.
Learn more about how to manage dehydration here.
If urine specific gravity is low, a person may have a health condition that causes them to drink a lot of fluid or affects their kidney function.
A doctor may:
- ask the person about any other symptoms
- ask if they have been drinking large amounts of fluid
- suggest further tests to identify the cause of the problem
A person does not need to think about raising their urine specific gravity. A doctor would need to find and correct the underlying cause. Addressing the cause can help normalize the specific gravity.
Learn more about what happens if you drink too much fluid.
Urine contains solutes, or soluble particles. If it did not contain these particles, urine specific gravity would be 1.000, the same as water. Urine specific gravity measures how many particles are in a person’s urine compared with water.
The higher the number of particles in the urine, the higher the urine specific gravity.
If there are more particles than in water, urine will be denser, and the specific gravity will be higher. Particles present in the urine may include glucose, proteins, and ketones.
Learn about mucus in the urine here.
The test should not be uncomfortable and is usually brief. The doctor might request multiple urine specific gravity tests in one day to see how well the kidneys compensate for a possible problem.
The doctor may ask the person to collect a “clean catch” specimen to prevent contaminating the sample with bacteria from the skin.
To collect a clean catch specimen, a person should:
- Wash their hands and have the container ready for use, taking care to touch only the outside.
- Clean the urethral area.
- Begin urinating, then hold the container under the stream.
- Collect at least 1 ounce (oz), or 30 milliliters (ml), of urine, as shown on the side of the container.
- Replace the cover securely, touching only the outside of the pot.
- Wash the hands thoroughly and follow any instructions for delivering the pot to the doctor or a lab for testing.
A laboratory technician performs the test using a refractometer, which projects light into the sample and helps determine the density of the urine.
Learn more about the kidneys and how they work here.
Preparation for a urine specific gravity test is typically straightforward.
Before the test, the doctor may ask the person to stop taking certain medications or to avoid some foods, such as beets and rhubarb, as these may affect the results.
People should also let their doctor know if they have recently received intravenous contrast dye for a medical test.
Learn more about how foods and medications can affect urine here.
A urine specific gravity test is a straightforward test that looks at a person’s urine concentration. The person will need to collect a clean urine sample for testing in a laboratory.
Urine concentration can show how well the kidneys are functioning or indicate an underlying condition that is altering the body’s fluid status.
A urine specific gravity test cannot diagnose a condition, but it can play a role in identifying underlying health conditions ranging from kidney failure to dehydration.