Urine specific gravity

Urine specific gravity measures the density (mass per unit volume) of the urine relative to the density of the water.

The urine specific gravity is a measure of the density of urine compared with the density of water, which is 1.000. It shows the concentration of all solid substances dissolved in the urine.

Therefore, the density of the urine is dependent on the amount and size of the dissolved substances present in a urine sample. These substances include glucose, proteins and electrolytes.

If specific gravity is high means that the urine is more concentrated while if it is low means that the urine is more diluted.

Infants own immature kidneys and they are not able to concentrate urine as effectively as mature kidneys. For this reason, the specific gravity in infants is lower.

Urine specific gravity test may be a help to diagnose early sign of a renal disorder, when values are out of range.

A high specific gravity means that the kidneys are putting out very concentrated urine. There are two main reasons for this matter:

• Dehydration (the most common cause).
• An increase in ADH (Antidiuretic Hormone) secretion due to trauma, stress or drugs.

A low specific gravity urine is indicative of dilute urine (very few dissolved substances). It may be due to:

• Excessive intake of fluids (water).
• Diuretics.
• ADH (Antidiuretic Hormone) deficiency that may be caused by suffering diabetes insipidus.

Some patients may have a fixed specific gravity around 1.010 that does not change even when the patient becomes dehydrated. This is a sign that the kidneys have lost their ability to concentrate urine. The 1.010 value is not casual. It is the same density of the blood plasma.

This test is performed along with other urine tests in what is called a urinalysis. The patient must collect a sample of the urine in a specific container using a special kit. This sample will be sent to the lab for the analysis.

The urine gravity test is measured frequently using reagent strips or dipsticks. The dipstick may comprise a reagent or chemical pad. It reacts (change color) when it is dipped into the urine specimen.

In laboratories, there are automated machines that perform the analysis very fast, but it is also possible to buy strips that let you analyze a urine sample at home. They comprise up to 10 or more different reagents or chemical pads to perform different urine tests at the same time.

Specific gravity may be also measured at labs with other procedures. The most common ones are:

• Urinometer: A test tube is filled with 10 or 20 ml of urine and then is placed in distilled water at 68-72ºF (20-22ªC). With a calibrated scale the specific gravity can be read depending on how much the test tube float in the distilled water.
• Refractometer: Only a drop of urine is needed. A monochromatic light is led to the drop and the specific gravity can be measured depending on the angle of refraction of the light.

The normal range for urine specific gravity is:

• Adults: 1.005 - 1.030
• Infant up to 2 years: 1.001 - 1.008
• Newborns: 1.012

Elderly people and children usually have lower values tan adults.

These values depend on the laboratory or the procedure used and there may be small difference from lab to lab. Each laboratory must establish its own normal range.

A high urine specific gravity value (above 1.030) may be due to:

• Congestive heart failure
• Dehydration
• Situations where the patient become dehydrated
  • Vomiting
  • Diarrhea
  • Excessive sweating
• Adrenal gland dysfunction
• Glycosuria (glucose in the urine)
• Proteinuria
• Mellitus diabetes

A low urine specific gravity value (below 1.005 and close to 1.000) may be due to:

• Diabetes insipidus
• Kidney failure
• Interstitial nephritis
• Excessive water intake
• Pyelonephritis
• Diuretics intake

Specific gravity values close to 1.010 in successive tests and in different hydration states may be due to:

• Chronic nephritis when the concentration power of renal tubules is low
• ADH (Antidiuretic Hormone) deficiency
• Renal artery stenosis

Specific gravity may be falsely high:

• If glucose or proteins in the urine are elevated.
• If some type of dye has been previously used in the urine for diagnostic purposes.
• If the sample is removed from the refrigerator.

Some drugs may increase the urine specific gravity and lead to a false positive such as Dextran, Mannitol or radiopaque contrast media used in X-rays of the urinary tract.

A high alkaline urine (urine with a high pH) may cause a low reading of urine specific gravity if reagent strip test is used.

The urine can be classified depending on the specific gravity into:

• Hyposthenuria: Specific gravity < 1.007 (dilute urine)
• Isosthenuria: Specific gravity between 1.008 y 1.012 (around 1.010)
• Hypersthenuria: Specific gravity > 1.012 (concentrated urine)