Urine specific gravity (USG)

Urine 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.

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 1.012 to 1.030

Adults generally have a specific gravity in the range of 1.000 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 excess of antidiuretic hormone caused by Syndrome of inappropriate antidiuretic hormone. 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, pyelonephritis, diabetes insipidus, acute tubular necrosis, interstitial nephritis, and excessive fluid intake (e.g., psychogenic polydipsia).

What Abnormal Results Mean

Increased urine specific gravity may be due to conditions such as:

Adrenal glands do not produce enough hormones (such as Addison disease)

Glucose in the urine

Heart failure

High sodium level in the blood

Loss of body fluids (dehydration)

Narrowing of the kidney artery (renal artery stenosis)

Shock

Syndrome of inappropriate ADH secretion (SIADH)

Decreased urine specific gravity may be due to:

Damage to kidney tubule cells (renal tubular necrosis)

Diabetes insipidus

Drinking too much fluid

Kidney failure

Low sodium level in the blood

Severe kidney infection (pyelonephritis)

1.       Krishnan A, Levin A. Laboratory assessment of kidney disease: glomerular filtration rate, urinalysis, and proteinuria. In: Yu ASL, Chertow GM, Luyckx VA, Marsden PA, Skorecki K, Taal MW, eds. Brenner and Rector's The Kidney. 11th ed. Philadelphia, PA: Elsevier; 2020:chap 23.

2.       Riley RS, McPherson RA. Basic examination of the urine. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 29.

3.       Villeneuve P-M, Bagshaw SM. Assessment of urine biochemistry. In: Ronco C, Bellomo R, Kellum JA, Ricci Z, eds. Critical Care Nephrology. 3rd ed. Philadelphia, PA: Elsevier; 2019:chap 55.