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Urinalysis
Urine specific gravity
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The ratio of the density of urine to that of water, which reflects the concentrating and diluting ability of the kidney. Isosthenuria corresponds to a fixed value near 1.010, the specific gravity of plasma ultrafiltrate.
Reference interval
| Conventional (ratio) | SI (ratio) | |
|---|---|---|
| Adults | 1.003–1.03 ratio | 1.003–1.03 ratio |
Specimen: Fresh random urine (first morning sample preferred) · Conventional × 1 = SI
Causes of high values
Dehydration, vomiting and diarrhoea, heart failure, SIADH, glycosuria, proteinuria, radiographic contrast media, high-dose dextran.
Causes of low values
Diabetes insipidus (central or nephrogenic), excess water intake, chronic kidney disease with loss of concentrating ability, acute tubular necrosis, diuretic use, hypercalcaemia, hypokalaemia.
Pre-analytical and method notes
The usual range is about 1.003 to 1.030 and the measurable range is wide because it depends on hydration. Osmolality is a more accurate measure of concentration, since specific gravity is raised by glucose, protein and contrast media; dipstick readings are approximate and affected by pH. Reference intervals vary by laboratory, method and population; use the interval printed on the report.
References
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Elsevier — Tier 3 (textbook)
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- Brenner and Rector's The Kidney. Elsevier, 11th edition (2019) — Tier 3 (textbook)
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