urinespecific

Urine specific gravity (USG) is a measure used to assess the concentration of urine. It compares the density of a urine sample to the density of distilled water, with higher values indicating more solutes and a more concentrated urine.

Measurement: Common methods include refractometry, which measures refractive index and is temperature-compensated, and urinometry, which uses a hydrometer. Dipstick tests provide rough estimates but are less precise, and results can be affected by temperature and assay interferences.

Normal range: In healthy adults, USG typically ranges from about 1.005 to 1.030; morning urine tends to be more concentrated. Values vary with hydration status, salt and protein intake, and kidney function.

Interpretation: Low USG (<1.010) suggests dilute urine due to overhydration, diuretics, or impaired concentrating ability. High USG (>1.020-1.030) indicates concentrated urine, seen with dehydration, reduced renal perfusion, or a high solute load such as glucose or protein in urine. Persistent abnormalities may prompt evaluation for diabetes insipidus, kidney disease, or other disorders affecting kidney concentrating capacity.

Relation to osmolality and limitations: USG correlates with urine osmolality but is not a direct measure of solute concentration; different solutes affect SG differently. SG can be influenced by very high glucose or protein, radiographic contrast media, or extreme temperatures; osmolality measurements may be preferred when precise quantification is needed.

See also: Urine osmolality, Urinalysis, Hydration status.