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Specific gravity (SG), the ratio of the mass of a solution compared to the mass of an equal volume of water, is an estimate of the concentration of substances dissolved in the solution.
Urine SG can be used to assess the kidneyâ€™s ability to concentrate or dilute urine. However, because protein, glucose, and contrast dye have molecular masses that are relatively large compared to other major components of urine (eg, sodium, chloride, potassium), they disproportionately affect SG. In these cases, urine osmolality is a better measure of urine concentration.
As a partial assessment of the kidneyâ€™s ability to concentrate urine
Low specific gravity (SG) (1.001-1.003) may indicate the presence of diabetes insipidus, a disease caused by impaired functioning of antidiuretic hormone (ADH). Low SG also may occur in patients with glomerulonephritis, pyelonephritis, and other renal abnormalities. In these cases the kidney has lost its ability to concentrate due to tubular damage.
High SG may occur in patients with adrenal insufficiency, hepatic disease, congestive heart failure, or in patients experiencing excessive water loss due to sweating, fever, vomiting, or diarrhea.
Urine with contrast dye, glucose, or excessive protein should not be evaluated with this test.
Urine osmolality is a better measure of urine concentration.
1. Schumann GB, Schweitzer SC: Examination of urine. In Clinical Chemistry, Theory, Analysis and Correlation. Edited by LA Kaplan, AJ Pesce. Third edition. St. Louis, Mosby-Year Book Inc. 1996 pp 1118-1119
2. Modern Urine Chemistry (Manual). Edited by HM Free. Bayer Corp, Eighth printing. 1996 pp 36-37