Specific Gravity

Urinary specific gravity (SG) is a measure of the concentration of solutes in the urine. It measures the ratio of urine density compared with water density and provides information on the kidney’s ability to concentrate urine. A urinary specific gravity measurement is a routine part of urinalysis. ^{[1, 2, 3]} The reference range is 1.005-1.030. ^[3]

Conditions associated with a high urine specific gravity include the following ^{[1, 4]} :

• Volume loss (dehydration, vomiting, diarrhea, fever)

• Hepatorenal syndrome

• Heart failure

• Renal artery stenosis

• Shock

• Syndrome of inappropriate antidiuretic hormone

Conditions associated with a low urine specific gravity include the following ^{[1, 4]} :

• Diabetes insipidus

• Renal failure

• Pyelonephritis

• Glomerulonephritis

• Psychogenic polydipsia

• Malignant hypertension

Specifics for collection and panels are as follows ^{[1, 4]} :

• Specimen type: Urine

• Container: Plastic urine specimen container

• Collection method: Random urine sample

• Specimen volume: 20 mL

• Panels: Urinalysis

• Other instructions: Keep specimen cool; high temperatures alter the result

• Related tests: Urine osmolality

Description

Urinary specific gravity is a measure of the concentration of solutes in the urine and provides information on the kidney’s ability to concentrate urine. Urine osmolality is a more specific test for urine concentration; however, urine specific gravity measurement is a routine part of urinalysis.

Indications/Applications

Indications for testing urine specific gravity include the following:

• Complicated urinary tract infection (eg, pyelonephritis)

• Hypernatremia

• Hyponatremia

• Polyuria

Considerations  ^[4]

The following may increase urine specific gravity and should be discontinued before testing:

• Dextran

• Sucrose

• Intravenous contrast dye

A study by Wilson indicated that body mass index (BMI) and lean body mass (LBM) affect specific gravity. The report found that in persons in the fifth quintile for BMI or LBM, the odds of having an elevated specific gravity were 2.00-3.68, compared with individuals in the first quintile. ^[5]