127 Urine specific gravity (USG)
Urine specific gravity (USG) compares the density of urine with that of water and reflects the concentration of dissolved substances in urine. It shows how well the kidneys concentrate or dilute urine according to the body's water balance, under the control of antidiuretic hormone (vasopressin). When the body needs to save water, urine becomes concentrated and USG rises; when there is excess water, urine is dilute and USG falls. It is a routine part of urinalysis.
Clinical significance
Low USG (dilute urine)
Large fluid intake, diuretics, diabetes insipidus (lack of or resistance to antidiuretic hormone), chronic kidney disease with loss of concentrating ability, primary polydipsia. A USG fixed near 1.010 regardless of fluid intake suggests advanced kidney damage.
High USG (concentrated urine)
Dehydration (fever, sweating, vomiting, diarrhoea), heart failure, inappropriate ADH secretion (SIADH), glucose in urine (uncontrolled diabetes), protein in urine, contrast agents after imaging.
Specific gravity is affected by large molecules (glucose, protein, contrast); urine osmolality is a more accurate measure of concentration when precise assessment is needed.
Excessive thirst and passing large amounts of urine, very dark urine despite drinking, or swelling and reduced urine output need medical assessment.
References
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 12 May 2024